LIASSIC ICHTHYOSAURS. 45 



of coeval fishes have been found within the costal cage of the fossil specimens in the 

 situation where the stomach may be judged to have have been. Small, hard, and un- 

 digested bodies, containing fish bones and scales, and bearing impressions of the folded 

 surface of the intestinal membrane, have received the name of " coprolites."^ 



Such were the air-breathers which governed the seas of our planet from the Liassic to 

 the Cretaceous period inclusive. At the later epoch the Ichthyopterygians became extinct, 

 and appear to have been superseded by the Mosasaurians. In these the vertebrae 

 have become proccelian ; their modified dentition both as to position and attachment is 

 continued on in existing Lizards, but the liiiihs were fins. The transition, if there was 

 such, is, however, abrupt, and the links are, as yet, unknown which connected the 

 Tertiary cetaceous Zeuglodonts with antecedent whale-like reptiles. 



The Cretaceous Ichthyosaurus canqryhdon - retains the characters of its order as 

 definitely as they are shown in the species of the ]\Iuschelkalk or Lias ; and the com- 

 mencement of this type of Reptile seems to be as abrupt as its close. Much remains 

 to be, and may be, discovered indicating the antecedent forms which linked on more 

 closely the Ichthyopteryyia with earlier air-breathing vertebrates. But with later ones 

 there is no evidence of transitional alliance ; they seem to have passed away under the 

 type of structure which I next proceed to explain as far as study of the fossil remains has 

 made it known to me. 



B. Osteology. 



a. Bones of the TrunJc. — In the vertebrse (Tabs. XVII and XVIII), according to the 

 regions of the column, are to be noted: the centrum (c) neurapophyses {n), neural spine 

 (n«), pleurapophyses {pi), haemapophyses {h), haemal spine {hs), zygapophyses {z, z), dia- 

 pophyses (rf), parapophyses {p), and hypapophyses [hy). Some of these are autogenous, 

 others exogenous parts. 



The centrum is more or less antero-posteriorly compressed (Tab. XVIII, figs. 1 to 

 7, 9, 13), with concave terminal articular surfaces (ib., fig. 6) not intercommunicating; on 

 each side of the shallow myelonal canal (ib., figs. 2 and 4, »?) is the deeper, usually 

 triangular, articular surface {np) for the neurapophyses (w). These, in each vertebra, converge, 

 and, save in the atlas (Tab. XVII, fig. 1, «), coalesce at their summits with each other and 

 with the neural spine (ib. figs. 2, 3, 4, n, ns). In most Fishes the neural arch coalesces 

 with the centrum, as in Cetaceans ; its separate state is a saurian, chiefly crocodilian, 

 modification ; it is such in the Ichthyosaurs, and adds to the power of inflecting the spine 

 vertically, as in the specimen (Tab. XXV, fig, 2).* 



^ BucKLAND (Dr. Wm.), "Discovery of the Faeces of the Ichthyosaurus," 'Trans, of the Geological 

 Society,' 2nd series, vol. iii, 1835. 

 « Vol. I, p. . 

 * More extreme and abrupt vertical flexures, shown in two specimens in the British Museum, may be 



