serrated on both sides, and sometimes of immense size (five or six inches in length). In Otodus (fig. 302) the teeth are not denticulated at their edges, and they have a secondary tooth

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Fig. 301.-Oxyrhina xipho- Fig 302.-Otodus obliquus. Fig. 303.—Carcharodon prodon. Eocene. ductus. Miocene.

Miocene.

at each side of the base. The teeth of Oxyrhina (fig. 301), lastly, are large and compressed, differing from those of Otodus chiefly in wanting the lateral projections at the base. Upon the whole, the deposits which have yielded the greatest abundance of the teeth of these extinct Sharks, are the Upper Greensand (Cretaceous) and the London Clay (Eocene Tertiary). c. Batides.--This group includes the Rays and Skates, and is distinguished by the fact that the branchial apertures are placed

on the under surface of the body, forming two rows of openings a little behind the mouth. In the typical members of the group, the body is

flattened out so as to form a kind of disc (fig. 304), the greater part of which is made up of the enormously developed pectoral fins. Upon the upper surface of the disc are the eyes and spiracles; upon the lower surface are the nostrils, mouth, and branchial apertures. The flat

Fig. 304.-Batides. Raia marginata, one of the Skates. Reduced one-sixth. (After Gosse.)

tened bodies of the Rays, however, must be carefully distinguished from those of the Flat-fishes (Pleuronectide). In

the former, the flat surfaces of the body are truly the dorsal and ventral surfaces. In the latter, as before remarked, the body is flattened, not from above downwards, but from side to side, and the head is so twisted that both eyes are brought to one side of the body.

The tail in the Rays is long and slender, usually armed with spines, and generally with two or three fins (the homologues of the dorsal fins). The mouth is paved with flat teeth of a more or less rhomboidal shape. The integument is often furnished with placoid structures of a peculiar shape, consisting of oval or rounded osseous discs, from the centre of each of which springs a curved spine of dentine. The tail also is sometimes armed with a doubly-serrated defensive spine.

The Rays are known in the fossil condition by their flat crushing teeth mainly, but also by their fin-rays, bony discs, and defensive spines. The earliest trace of the Rays is found in the Carboniferous Rocks, where occurs the doubly-serrated spine which is referred to the genus Pleuracanthus (fig. 296, 1). In this singular form, however, the spine seems to have been inserted at the back of the head, instead of in the tail, as in the living Sting-rays. In the Jurassic Rocks occur the remains of Rays, which have been referred to the genera Squaloraia, Spathobatis, Arthropterus, &c.

In the Tertiary Rocks the remains of Rays are tolerably abundant, and consist almost exclusively of the dental plates. These consist (fig. 305) of generally flat plates, usually somewhat rhomboidal in shape, often placed close together and sometimes united laterally by sutures, so as to "form a kind of mosaic pavement on both the upper and lower jaws" (Owen). Most of the fossil forms belong to the genus Mylio batis, which comprises the living Eagle-rays. All the fossil species of this family belong to the Tertiary period.

ORDER IV.-DIPNOI (= Protopteri, Owen).-This order is a very small one, and includes only the singular Mud-fishes (Lepidosiren and Ceratodus); but it is nevertheless of great importance as exhibiting a distinct transition between the Fishes and the Amphibia. So many, in fact, and so striking, are the points of resemblance between the two, that until recently the Lepidosiren (fig. 306) was always made to constitute the lowest class of the Amphibia. The highest authorities, however, now

concur in placing it amongst the fishes, of which it constitutes the highest order. The order Dipnoi is defined by the following characters :-The body is fish-like in shape. There is a skull with distinct cranial bones and a lower jaw, but the notochord is persistent, and there are no vertebral centra, nor an occipital condyle. The exoskeleton consists of horny, overlapping scales, having the "cycloid" character. The pectoral and ventral limbs are both present, but have (in Lepidosiren) the form of awl-shaped, filiform, many-jointed organs, of which the former only have a membranous fringe inferiorly. The ventral limbs are attached close to the anus, and the pectoral arch has a clavicle; but the scapular arch is attached to the occiput. The hinder extremity of the body is fringed by a vertical median fin. The heart has two auricles and one ventricle. The respiratory organs are twofold, consisting on the one hand of free filamentous gills contained in a branchial chamber, which opens externally by a single vertical gill-slit; and on the other hand of true lungs in the form of a double cellular air-bladder, communicating with the esophagus by means of an air-duct or trachea. The branchia are supported upon branchial arches, but these are not connected with the hyoid bone; and in some cases, at any rate, rudimentary external branchia exist as well. nasal sacs open posteriorly into the throat.

The

Until lately the only known members of the order Dipnoi were the Lepidosiren paradoxa of South America and the Lepidosiren (Protopterus) annectens of Africa. No fossil also could be referred with any certainty to this order. Recently, however, there has been discovered a most remarkable fish in the rivers of Queensland (Australia), which is almost certainly referable to this order, and which throws great light upon several fossil forms. The organisation of this fish is so extraordinary, and its affinities with some of the extinct Ganoids are so numerous and important, that it will be well to quote at some length the description of it given by Dr. Albert Günther, one of the most eminent of living ichthyologists. The fish in question has been named the Ceratodus Fosteri, and it is known locally as the "Barramunda." It is said to attain a length of about six

feet, but its average length is about three feet. The Barra munda "is eel-shaped, but considerably shorter and thicker than a common eel, and covered with very large scales. The head is flattened and broad, the eye lateral and rather small, the mouth in front of the broad snout and moderately wide. The gill openings are a rather narrow slit on each side of the head. There are no external nostrils. The tail, which is of about the same length as the body without the head, is compressed, and tapers to a point, but it is surrounded by a very broad fringe, supported by innumerable fine and long fin-rays. There are two fore and two hind paddles, similar to each other in shape and size, and very different from the fins of ordinary fishes; their central portion being covered with a scaly skin, and the entire paddle surrounded by a rayed fringe. If we were to cut off the hind part of the tail of a fish, the piece would bear a strong resemblance to one of the paired paddles. The vent is situated in the median line of the abdomen between the paddles.

"In order to obtain a view of the inside of the mouth, it is necessary to slit it open, at least on one side. We then notice that there are a pair of nasal openings within and on each side of the cavity of the mouth. The palate is armed with a pair of large, long, dental plates, with a flattish undulated and punctated surface, and with five or six sharp prongs on the outer side, entirely similar to the fossil teeth described under the name of Ceratodus. Two similar dental plates of the lower jaw correspond to the upper, their undulated surface fitting exactly to that of the opposite teeth. Beside these molars, the front part of the upper jaw (vomer) is armed with two obliquely placed incisor-like dental lamellæ, which have no corresponding teeth in the lower jaw. As we know the kind. of food taken by the Barramunda, the use of these teeth is apparent. The incisors will assist in taking up or even tearing off leaves, which are then partially crushed between the undulated surfaces of the molars.

"The skeleton consists of a cartilaginous basis, in the form of a long tapering chord for the body and tail, and in that of a capsule for the head. No segmentation into separate vertebræ is visible in any part of the notochord; but it supports a considerable number of apophyses, the abdominal of which bear well-developed ribs, all being solid cartilaginous rods, with a thin sheath of bone. In the same manner no part of the braincapsule is ossified, but it is nearly entirely enclosed in thin bony lamellæ. This is also the structure of the appendages of the skull, as the mandible and the hyoid and scapulary arches.

From a study of the skull, it becomes apparent at once why in fossil teeth of Ceratodus nothing or very little of the bone attached to them has been preserved. These teeth rest on cartilage as well as on bone, the latter being a very thin and porous layer which could not be preserved, unless the progress of stratification had been going on with as little disturbance as in the Solenhofen Schiefer; but the matrix in which fossil Ceratodont teeth are found shows that it was formed under very different conditions, and it is certainly not of a nature to permit the supposition that thin porous lamellæ of bone would have been preserved entire.

"The structure of the skeleton reminds us much of that of the sturgeons, Chimæra, and especially of Lepidosiren; and of all the modifications by which it differs from these types, perhaps none is of greater interest than that observed in the paddles. The central part of the paddle, which we have found externally to be covered with scales, is supported by a jointed axis of cartilage extending from the root to the extremity of the paddle; each joint bears a pair of three- or two- or one-jointed branches. This is the case in the hind as well as fore paddles, and we are justified in supposing that those extinct Ganoids of which impressions of paddles with scaly centres have been. preserved, were provided with a similar internal skeleton."

Upon the whole, Dr. Günther concludes:-1. That the Barramunda is not generically separable from the almost exclusively Triassic genus Ceratodus, which was founded simply upon detached teeth; 2. That the Barramunda is very closely allied to certain of the Crossopterygious Ganoids, such as the Dipterus of the Old Red Sandstone, the chief difference being, that the tail of the latter is heterocercal; 3. That the order Dipnoi should be considered merely as forming a sub-order of the Ganoidei; 4. That the Ganoidei may be united with the Elasmobranchii into a single group, which may be termed Palaichthyes, and which is characterised by having a "heart with a contractile bulbus arteriosus, intestine with a spiral valve, and optic nerves non-decussating;" 5. That the Ganoidei are the Fresh-water Palaichthyes, and the Elasmobranchii are the Marine Palichthyes.

If the views of this high authority be ultimately adopted, it will have to be admitted that the Dipnoi, instead of being unknown in a fossil state, have enjoyed a vast antiquity, dating their existence from the Lower Old Red Sandstone.