The specimens consist of the jaws with teeth, vertebræ, and some bones of the extremities.[616] In one species, the vertebræ are biconcave; in the other, they are convexo-concave, and present a remarkable deviation from the recent Crocodilian type, namely, that they are placed in a reversed position,—the convex face of the vertebra being directed anteriorly, or towards the cranium, and the concavity posteriorly; the name of the genus, Streptospondylus (reversed-spine), denotes this peculiarity of structure. The bodies of three or four large convexo-concave cervical vertebræ, were discovered in the Tilgate strata many years since, and are described in my various works (Geol. S. E. p. 300); but no suspicion was then entertained of their belonging to this genus, although I had repeatedly compared them with the figures of the Honfleur crocodile,[617] the imperfect state of the processes obscuring their true characters. Professor Owen first detected the true character of these Wealden vertebræ, in a large cervical, six inches long (now in the British Museum), in which two oblique processes are preserved on the concave end of the bone, their flat, oblong, articular faces, are directed downwards and outwards,—a character which at once proves them to be the posterior pair, for the anterior oblique processes would be directed upwards and inwards.[618] Vertebræ of the same species occur in the Wealden of the Isle of Wight; and of another species in the Oolite at Chipping Norton, and in the Lias of Whitby.
[616] Oss. Foss. tom. v. p. 143, pl. viii. ix.
[617] Reduced figures of two of these vertebræ are given in [Lign. 206], figs. 5 and 7, p. 653.
[618] Brit. Assoc. Rep. 1841, p. 92. The position of the articulating surfaces of the oblique processes (zygapophyses) in more perfect specimens, subsequently discovered in various localities of the Wealden, established the existence not only of a saurian allied to the Streptospondylus, but also of other reptiles whose spinal column was wholly or in part made up of vertebræ which were convex in front and concave behind, as in the cervicals and anterior dorsals of certain mammalia. Some of these fossil vertebræ Dr. Melville has referred, with great probability, to the cervical region of the Iguanodon (see Petrif. p. 259); others cannot at present be satisfactorily assigned to any known genus of reptiles.
A concavo-convex caudal vertebra, with the relations of which I am unacquainted, was found in the same quarry in Tilgate Forest; a reduced outline of this unique fossil is given in [Lign. 206], fig. 1. The centrum is of a sub-cylindrical form, and the articular face in front is concave, and that behind, convex; with a chevron-bone that is anchylosed to the body of the vertebra, as in some of the caudals of the Mosasaurus, and terminates in an inferior spine (f.); the pair of anterior oblique processes remains; the neural spinous process is destroyed.
Cetiosaurus.—From a considerable number of vertebæ and bones of the extremities of some gigantic aquatic reptiles, discovered in the Oolite in various places in Oxfordshire, Northamptonshire, and Yorkshire, the present genus was established; the name being intended to indicate a distant general resemblance of these extinct Saurians to the Cetaceans.[619] The vertebra; differ from those of the Iguanodon in having their articular faces of a sub-circular form, and the body relatively short; the anterior face is nearly flat, and the posterior concave, in the dorsal vertebra;; but in the caudal both faces are concave, and have a well-defined elevated margin, which gives the body a deeply excavated character, easily recognizable. Vertebræ of this kind were among my earliest discoveries in the strata of Tilgate Forest. (Geol. S. E. p. 282.) Some specimens are eight inches in the transverse diameter of the articular face, and but four and a half inches in the antero-posterior length of the body.[620] The original animals are supposed to have been of aquatic, and probably of marine habits, on the evidence of the sub-biconcave structure of the vertebræ and of the coarse cancellous tissue of the long bones, which are destitute of a medullary cavity. They must have rivalled the modern whales in bulk, for some specimens indicate a length of forty or fifty feet; they are supposed to have had web-feet, and a broad vertical tail.[621]
[619] Proc. Geol. Soc. vol. iii. p. 457.
[620] The osteological characters of these remains, and the physiological relations of the original animals, are described in Brit. Assoc. Rep. 1841, pp. 94-102.
[621] Brit. Assoc. Rep. 1841, p. 102.