They have generally been placed amongst the Ganoids; but Professor Huxley has pointed out that they present, many of them, features by which they approximate closely to the Siluroids amongst the Teleosteans. The more important genera included in this sub-order are Cephalaspis, Pteraspis, Coccosteus, and Pterichthys.

Cephalaspis (fig. 292) is the type of the family of the Cephalaspida, and is readily recognised by the fact that the cephalic shield has its posterior angles produced into long "cornua,"

Fig. 292.-Cephalaspis Lyellii. (After Page.) Old Red Sandstone.

giving it the shape of a "saddler's knife." Besides these lateral cornua, there is a "posterior cornu" or spine, formed by a prolongation backwards of the hinder margin of the shield in the middle line. The orbits are approximated, and are placed nearly in the centre of the cephalic shield. No jaws or teeth are known, and the mouth was probably soft, and adapted for suction. The head-shield exhibits vascular canals, and shows very distinct bone-cells when examined in thin sections under the microscope. The body is covered with ganoid scales, and there is a well-marked dorsal fin. Pectoral fins have also been described, and the tail is clothed with a heterocercal fin. In the nearly allied Auchenaspis, the structure is very similar to the above, but there is no spine or "posterior cornu," and there is instead a neck-plate formed by an extension backwards from the cephalic shield. The Cephalaspide are mainly found in the Old Red Sandstone, the commonest species being C. Lyellii. Other species are found in the "passage-beds between the Silurian and Old Red, and the genus is not wholly unrepresented in the Upper Silurian deposits.

In the genus Pteraspis (fig. 293) the head is defended, as in Cephalaspis, by a shield or buckler, which is composed of several pieces firmly anchylosed. The shield consists of a central disc, the lateral angles of which are produced into short cornua,

whilst it is extended into a rostrum in front. The posterior spine is very small, and is attached to the disc as a separate piece. The orbits are situated laterally. The minute structure of the shield is very complex, consisting of three layers. The innermost layer is laminated, and is traversed by vascular canals. The middle layer is made up of polygonal cavities; and the outer layer is structureless or fibrous, and is finely striated or grooved. The body was covered with scales; but nothing is known of the nature of the fins. The genera Cyathaspis and Scaphaspis have been founded upon forms which have usually been placed under Pteraspis, and which differ in more or less essential points from the typical species of this genus. The genus Pteraspis, so far as yet known, comprises the most ancient of the fishes, commencing as it does in the earlier portion of the Ludlow formation (Upper Silurian). Other species are known in the Old Red Sandstone; but the genus appears to have entirely disappeared before the close of the Devonian period.

In the genus Coccosteus the head was protected by a great shield, the plates of which are covered with small hemispherical tubercles. There is also a ventral or "sternal" shield, which, according to Huxley, seems to have had no direct connection with the cephalic buckler. The mouth was furnished with a distinct lower jaw or "mandible," composed of two rami, carrying small teeth. The notochord was persistent, but the neural and hæmal spines of the vertebræ, and the rays of the dorsal and anal fins, are well ossified. A heterocercal tailfin was doubtless present as well. The genus Coccosteus is essentially Devonian; but a species has been discovered by M. Barrande in the Upper Silurian of Bohemia.

In the genus Pterichthys (fig. 294) are some very remarkable fishes, first discovered in the Old Red Sandstone by the late Hugh Miller, and nearly related in most respects to Coccosteus. The whole of the head and the anterior part of the trunk were defended by a buckler of large ganoid plates suturally united, those covering the trunk forming a back-plate and a breastplate articulated together at the sides. The rest of the body was covered with small ganoid scales. A small dorsal fin, a pair of ventrals, a pair of pectorals, and a heterocercal tail-fin were present. The form of the pectoral fins is the peculiar

characteristic of the genus. These were in the form of two long curved spines, somewhat like wings, covered by finelytuberculated ganoid plates. From their form they cannot have been of much use in swimming; but they probably, as suggested by Owen, enabled the animal to shuffle along the sandy bottom of the sea, if stranded at low water. All the species of Pterichthys are exclusively confined to the Old Red Sandstone.

SUB-ORDER G. STURIONIDE.-In this sub-order the skeleton is almost altogether cartilaginous, and the notochord is persistent. The exoskeleton is usually in the form of large ganoid plates, which are united into a shield over the head, but are detached over the body. Sometimes the exoskeleton is almost absent, and in no case is the mouth furnished with teeth. The tail is heterocercal.

This sub-order comprises the living Sturgeons, and is not known with certainty to have come into existence before the Eocene Tertiary, where it is represented by the Acipenser toliapicus of the London Clay. In the Lias, however, occur two species of the singular genus Chondrosteus, which have usually been referred here, and have been regarded as being most nearly allied to the Paddle-fishes (Spatularia) of North America. The skull, however, is more completely ossified than is the case with any living members of the Sturionida; and the true place of Chondrosteus must be regarded as uncertain.

CHAPTER XXX.

ORDERS Of Fishes-Continued.

ORDER III. ELASMOBRANCHII (= Selachia, Müller; Placoidei, Agassiz; Holocephali and Plagiostomi, Owen).-This order includes the Sharks, Rays, and Chimæræ, and corresponds with the greater and most typical portion of the Chondropterygida or Cartilaginous Fishes of Cuvier. The order is distinguished by the following characters :- The skull and lower jaw are well developed, but there are no cranial bones, and the skull consists of a single cartilaginous box, without any indication of sutures. The vertebral column is sometimes composed of distinct vertebræ, sometimes cartilaginous or sub-notochordal. The exoskeleton is in the form of placoid granules, tubercles, or spines. There are two pairs of fins, representing the limbs, and supported by cartilaginous fin-rays; and the ventral fins are placed far back near the anus. The pectoral arch has no clavicle. The heart consists of a single auricle and ventricle, and the bulbus arteriosus is rhythmically contractile, is provided with a special coat of striated muscular fibres, and is furnished with several transverse rows of valves. The gills are pouch-like.

In most of the above characters it will be seen at once that the Elasmobranchii agree with the Ganoid fishes, especially as regards the structure of the heart. The following points of difference, however, require more special notice :

I. The exoskeleton is what is called by Agassiz "placoid." It consists, namely, of no continuous covering of scales or ganoid plates, but of more or less numerous detached grains, tubercles, or spines, composed of bony matter, and scattered here and there in the integument. In the case of the Rays, these placoid ossifications often take a very singular shape, consisting of an osseous or cartilaginous disc, from the upper surface of which springs a sharp recurved spine, composed of dentine.

II. The gills are fixed and pouch-like, and differ very materially from those of the Bony and Ganoid fishes. In the case of the Sharks and Rays, the gill-pouches open upon the surface by a series of separate apertures, which are placed on the sides of the neck in the former, and on the under surface of the body in the latter. In neither is there any gill-cover or operculum, nor are there any branchiostegal rays. In one section of the order, however-viz., the Holocephali-though the internal structure of the gills is essentially the same as in

the Sharks, there is only a single branchial aperture or gill-slit externally, and this is protected by a rudimentary operculum and branchiostegal rays.

The order Elasmobranchii is divided into the two sub-orders of the Holocephali and Plagiostomi. The former comprises the living Chimara, characterised by the mouth being terminal, and there being only a single gill-slit. The latter comprises the living Port Jackson Shark, the true Sharks and Dog-fishes, and the Rays, characterised by having the mouth transverse and placed on the under surface of the head, whilst there are several apertures to the gills.

As regards their general distribution in time, the Elasmobranchii are nearly as ancient as the Ganoids. At the top of the Upper Ludlow Rocks, or at the close of the Upper Silurian epoch, there have been discovered the remains of undoubted Plagiostomous fishes, most nearly allied to the existing Port Jackson Shark (Cestracion Philippi). These remains consist chiefly of defensive spines, which formed the first rays in the dorsal fins, and upon these the genus Onchus (fig. 295) has been founded. Besides these there have been found portions of skin or "shagreen," with little placoid tubercles, like the skin of a living shark. These have been referred to the genera Sphagodus and Thelodus. They are the earliest known remains of Plagiostomous fishes, and with the exception of the few remains from the Lower Ludlow Rocks, they are the earliest known remains of fishes in the stratified series. The discovery of these remains, at that time the earliest known traces of Vertebrate life, is due to the genius of Sir Roderick Murchison, the author of 'Siluria.'

Most of the fossil Elasmobranchii belong to the division Cestraphori of Owen, so called because they are provided with the large fin-spines which are known to geologists as "ichthyodorulites." The two families of this division—the Cestracionts and Hybodonts-are largely represented in past time, the former chiefly in the Palæozoic period, the latter chiefly in the Mesozoic Rocks. Subjoined is an illustration of the "ichthyodorulites" and teeth of some of the Paleozoic Cestraphori.

The true Sharks are represented in the Mesozoic deposits (eg, by teeth of Notidanus in the Oolites); but they are chiefly Cretaceous and Tertiary. The teeth of Odontaspis, Galeocerdo, and Carcharodon, are good examples from the Eocene of the Isle of Sheppey. The true Rays are older than the true Sharks, the Carboniferous fossil Pleuracanthus being probably the spine of a Ray (fig. 296). Numerous remains of Rays, chiefly in the form of the pavement-like teeth, are known,