LIASSIC FORMATIONS. 87 



of their large and strong fore paddles against the scapular arch, to have crawled or dragged 

 themselves along with the belly resting on the ground. 



In outward form an Ichthyosaur (PI. XXVIII, fig. 1) resembled a huge predatory 

 abdominal fish, with a longer tail and smaller or shallower tail-fin ; scaleless, moreover, 

 being clothed by a smooth, probably finely wrinkled, skin, 1 something like that of the 

 whale-tribe. 



The mouth was wide and the jaws long, armed, as a rule, with numerous pointed and, 

 in some species, trenchant teeth. Masses of comminuted bones and detached ganoid scales 

 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." 2 



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 and appear to have been superseded by the Mosasaurians.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 vertebras 

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

 continued on in existing Lizards, but the limbs 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 campylodon 3 retains the characters of its order as 

 definitely as they are shown in the species of the Muschelkalk 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 Ichthyopterygia 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 Trunk. — In the vertebras (Pis. XXI and XXII), according to the 

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

 {ns), pleurapophyses (pi), haemapophyses {h), haemal spine (J IS ), zygapophyses (», z), dia- 

 pophyses (d), parapophyses (p), and hypapophyses (%)• Some of these are autogenous, 

 others exogenous parts. 



1 " Description of some of the Soft Parts with the Integument of the Hind Fin of the Ichthyosaurus, 

 &c," 'Trans, of the Geological Society,' 2nd series, vol. vi, 1840, p. 199, pi. xx. 



8 Bucklaxd (Dr. Wm.), "Discovery of the Faeces of the Ichthyosaurus," 'Trans, of the Geological 

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



s ' Monograph on the Fossil Reptilia of the Cretaceous Formations ' (Palseontographical Society's 

 volume, 4to, 1851). 



