b. Spirulirostra.-The shell (fig. 265) in this singular genus consists of a chambered portion or phragmacone coiled into a spiral, the volutions of which are separated. This is lodged in a pointed calcareous portion or "rostrum." The only known species of this genus is found in the Miocene Tertiary.

Fig. 265.-Spirulirostra Bellardii. Miocene Tertiary.

c. Beloptera.-Shell consisting of a nearly straight chambered portion or "phragmacone," perforated by a siphuncle, and lodged in a pointed calcareous rostrum which is furnished with lateral wings. Two species only of this genus are known, both from the Eocene Tertiary.

d. Belemnosis.—This genus has been founded for the recep tion of an Eocene fossil closely resembling Beloptera, but differing in not possessing any lateral expansions.

FAM. 5. SPIRULIDE.-Shell nacreous, discoidal, composed of volutions which are not in contact with one another. The shell is divided into a series of air-chambers by curved shelly partitions, pierced by a ventral tube or "siphuncle." The entire shell corresponds with the "phragmacone" of the skeleton of the Belemnites. Spirulirostra and Beloptera are often referred to this family; but if these be placed in the Sepiada, the family of the Spirulida is then without any known fossil representative.

FAM. 6. BELEMNITIDE.-Shell internal, composed of a conical chambered portion (" phragmacone "), with a marginal or ventral siphuncle, lodged in a cylindrical fibrous "guard," and produced in front into a thin horny or shelly plate or "pen" (the "pro-ostracum "). The Belemnitida are exclusively confined to the Secondary Rocks, ranging from the top of the

Trias to the Chalk. The following are the more important genera belonging to this family :

a. Belemnites.-The skeleton of the Belemnite consists of a sub-cylindrical, longer or shorter, fibrous body (fig. 266), which is termed the "rostrum " or "guard.' The length of the guard varies very much in different cases, and it is the part of the Belemnite which is most commonly found in a fossil condition. At the front or broad end, the guard is hollowed out into a conical excavation, which is termed the "alveolus." Within the alveolus, in perfect specimens, is contained the "phragmacone." This consists of a conical series of chambers, separated from one another by curved shelly partitions or septa, which are perforated by apertures for the passage of the "siphuncle." The siphuncle traverses the middle of the ventral wall of the phragmacone, and the whole series of chambers is enclosed in a thin shell-wall (the "conotheca" of Huxley). Anteriorly the conotheca or investment of the phragmacone is prolonged forwards into a horny or shelly plate, which corresponds with part of the "pen" of the Calamaries, and which is termed the "pro-ostracum" (fig. 266, r). The form of the "proostracum" varies greatly in different cases, and it affords important characters in the discrimination of specific and generic forms in the Belemnitide. Owing, however, to its extreme tenuity, it is very rarely found preserved in a fossil condition, and its value to the working palæontologist is thus greatly reduced.

r

Horny or shelly pen or pro

ostracum;"Chambered "phragmacone" in its cavity (a) or "alveolus;"g" Guard."

Not only is the internal skeleton of the Belemnite known, but various specimens have been discovered, from which much has been learnt as to other points

of its anatomy. Thus we know that the body was furnished with lateral fins, that there were eight arms and two longer "tentacles," that the suckers were provided with horny hooks, that there was a large ink-sac, and that the mouth was armed with horny mandibles.

The following table of the sections and sub-sections of the species of the genus Belemnites is the one given by Dr S. P. Woodward:

Section I. Acœli.

Without dorsal or ventral grooves.

Sub-section 1. Acuarii.

Without lateral furrows, but often channelled at the extreme point. (Ex. B. acuarius. Lias.)

Sub-section 2. Clavati.

With lateral furrows. (Ex. B. clavatus. Lias.)

Section II. Gastrocoli.

Ventral groove distinct.

Sub-section 1. Canaliculati

No lateral furrows.

Sub-section 2. Hastati.

(Ex. B. canaliculatus. Inf. Oolite.)

Lateral furrows distinct. (Ex. B. hastatus. Oolite.)

Section III. Notocoli.

With a dorsal groove, and furrowed on each side. (Ex.
B. dilatatus. Neocomian.)

The species of the genus Belemnites range from the top of the Trias, where the earliest forms appear, to the Upper Greensand, in which the genus finally disappears. The species are most numerous in the Jurassic Rocks, and often occur in the greatest abundance in particular beds or particular localities. It would seem not improbable that the genus Belop tera, before noticed, should be referred to the Belemnitida, and the genus Belemnosepia (or Geoteuthis), formerly referred to the Teuthida, appears to be almost certainly referable here.

b. Belemnitella.-In this genus (fig. 267) the skeleton is very similar in its general arrangement to that of Belemnites; but there is a straight fissure in the guard, at its upper end, on the ventral side of the wall of the alveolus. The species of this genus are Fig. 267.— exclusively Cretaceous, and are only found in the Belemnitella upper portion of this formation, ranging from the Upper Greensand to the Chalk.

mucronata.

Chalk.

c. Belemnoteuthis.-" Shell consisting of a phrag macone, like that of the Belemnite; a horny dorsal pen with obscure lateral bands; and a thin fibrous guard, with two diverging ridges on the dorsal side. Animal provided with

arms and tentacles of nearly equal length, furnished with a double alternating series of horny hooks, from 20 to 40 pairs on each arm; mantle free all round; fins large, medio-dorsal." -(Woodward.) Only one species is known, from the Oxford Clay (Middle Oolites). High authorities, such as Owen and D'Orbigny, question the validity of this genus, and regard it as being founded upon specimens of Belemnites.

d. Xiphoteuthis.-Guard narrow and cylindrical, containing a very long, deep-chambered, narrow phragmacone. Proostracum greatly developed (nearly a foot in length), very narrow at its base, widening out anteriorly, and finally terminating in a pointed apex. Only a single species is known, from the Lias.

CHAPTER XXVII.

SUB-KINGDOM VERTEBRATA.

THE sub-kingdom Vertebrata may be shortly defined as including animals in which the body is composed of a succession of definite segments, arranged along a longitudinal axis; the main masses of the nervous system (brain and spinal cord) are situated along the dorsal surface of the body, and are completely shut off from the general body-cavity. The limbs are never more than four in number, and are always turned away from that aspect of the body upon which the main masses of the nervous system are situated. In all, the nervous axis is primitively supported by a cellular rod, which is termed the "notochord;" but in most the notochord is replaced in the adult by the bony axis known as the "spine" or "vertebral column."

The past existence of Vertebrate animals is chiefly recognised by the preservation of their hard structures. These hard structures are of two kinds-some belonging to the internal or true skeleton (endoskeleton), others being of the nature of horny or bony plates, scales, or appendages of various kinds, developed in the integument (exoskeleton). The nature of the exoskeleton in the Vertebrates differs very much in different cases, and it will be considered when treating of the separate groups. It will be well, however, to give an extremely general and brief view of the structure of the endoskeleton, taking for this purpose a Mammal as a typical form. In this way the student will be enabled readily to trace the modifications of the skeleton in the lower forms, and will without

difficulty comprehend the terms which are necessarily employed in the definitions of the various groups. It may be added here, before proceeding further, that it does not seem requisite to treat the Vertebrata with the same fulness as the Invertebrata. The fossil remains of Vertebrates are in many cases of the highest theoretical interest, but they come much less frequently under the notice of the ordinary student than do the remains of the Invertebrates. No practical study, also, of the fossil Vertebrates can be carried out without a considerable acquaintance with Comparative Osteology. Lastly, the remains of Vertebrate animals generally occur in such a fragmentary condition that a sufficient series of specimens for profitable study can rarely be obtained, except under peculiarly favourable circumstances, in special cases, or where access can be had to a first-rate museum. For these and other reasons it is thought enough, in a treatise intended for the working palæontologist, to give a general account of each class of the Vertebrata, with definitions of the orders, and a brief notice of the leading forms of each. Only in cases of special interest will any details of a more minute character than the above be given.

The skeleton of the Vertebrata may be regarded as consisting essentially of the bones which go to form the head and trunk on the one hand (sometimes called the "axial" skeleton), and of those which form the supports for the limbs ("appendicular" skeleton) on the other hand. The bones of the head and trunk may be looked upon as essentially composed of a series of bony rings or segments, arranged longitudinally, one behind the other. Anteriorly these segments are much expanded, and likewise much modified, to form the bony case which encloses the brain, and which is termed the cranium or skull. Behind the head the segments enclose a much smaller cavity, which is called the "neural" or spinal canal, as it encloses the spinal cord; and they are arranged one behind the other, forming the vertebral column. The segments which form the vertebral column are called "vertebræ," and they have the following general structure :-Each vertebra (fig. 268, A) consists of a central piece, which is the fundamental and essential element of the vertebra, and is known as the "body" or "centrum” (c). From the upper or posterior surface of the centrum spring two bony arches (n n), which are called the "neural arches" or "neurapophyses," because they form with the body a canal-the "neural canal "-which encloses the spinal cord. From the point where the neural arches meet behind, there is usually developed a longer or shorter spine, which is termed the "spinous process" or "neural spine" (s). From the neural arches