These spines are generally capable of being elevated and depressed, and not only serve the purpose of defence, but, in many instances, afford support and protection to the soft rays of the fin; forming, as it were, a moveable mast, by which the sail can be spread out or lowered at pleasure.
In illustration of this subject, I would first direct attention to the beautiful fossil, figured [Lign. 188], fig. 1, which was discovered in the Chalk near Lewes, and is figured, of the natural size, Foss. South D. tab. xxxix. This ray, or spine, belongs to one of the Cestraciont fishes (Ptychodus), whose teeth are so abundant in the Chalk, and will presently be described. It is composed of fourteen thick, flat, osseous rods, or strands, intimately united together, with longitudinal furrows or sutures on the surface. The anterior margin is embossed, and the projections form on the sides wide, rounded ribs, and transverse depressions. Towards the base of the posterior part, there are large osseous fibres inserted vertically and obliquely, which appear to have been processes of attachment. The rods, or plates, are parallel With the posterior margin, and each terminates in a rounded extremity, or boss, on the front edge of the spine. This ray is wider at its base than at the superior part the anterior margin is oblique, and the posterior straight. The surface, where entire, is covered with a dense osseous substance, which is finely engrailed.[515]
[515] This specimen is figured in Poiss. Foss.; but it is represented too short, from the two portions being drawn as if they were connected, without any interval between them, as in [Lign. 188]. It is in the British Museum. See Petrifactions, p. 450.
Lign. 188. Dorsal Rays of Sharks. Sussex.
| Fig. | 1.— | Ptychodus spectabilis. 1/5 nat. Chalk. Lewes. |
| 1a.— | Portion of a Dorsal Ray, with oblique serrated sutures.Chalk. Lewes. | |
| 2.— | Ptychodus Gibberulus. 1/5 nat. Chalk. Lewes. | |
| 3.— | Hybodus subcarinatus. nat. Weald. Tilgate Forest. |
In 1850 I discovered in the Plastic Clay of Castle Hill Newhaven, a dorsal fin of Ptychodus, with eight vertebræ. A nearly entire fin-ray of this species, three feet in length, has recently been discovered by Mr. Charles Potter, of Lewes, in the Chalk near that town. The remains of another ray, of equal proportions, were found near it; and these dorsal spines might have belonged to the same individual, for there are no reasons to forbid the supposition that the Ptychodus had two dorsal fin-rays. The length of these spines necessarily indicates a very large fish.
A smaller species of Ichthyodorulite, also found in the Lewes Chalk, is distinguished from P. spectabilis by its osseous plates contracting towards their extremities, and terminating more suddenly on the front margin, producing gibbosities less acute and more distant than in P. spectabilis; this species is named P. gibberulus: see [Lign. 188], fig. 2.[516]
[516] This fossil is figured of the natural size, Foss. South D. pl. xl. fig. 3.