44 BRITISH FOSSIL REPTILES. 



width of the back of the head with the front of the chest, shares witli the Wiiale a 

 resemblance to Fishes, but pushes the Ulceness closer in the greater number and less 

 length of the vertebrae, and in the indication of the main joints of the backbone being 

 elastic bags filled with fluid, occupying the intervertebral spaces of the biconcave 

 centrums, as in Fishes, Labyrinthodonts, and modern perennibranchiate Batrachians. 



Being cold-blooded, and with a small brain needing a much less supply of oxygen for 

 its work, the Ichthyopterygians, like Fishes, had this advantage over Whales, that their 

 stern-propeller could have the form best adapted for a swift straightforward course 

 through the water.^ The horizontality of the tail-fin of the Whale tribe relates to their 

 need, as large-brained, warm-blooded air-breathers, to have easy and speedy access to 

 atmospheric air. Without the means of displacing a mass of water in the vertical 

 direction by such broad tail-fin the head of the Whale could not be brought with the needed 

 rapidity to the surface for the purpose of breathing. Nevertheless the Cetaceans are 

 restricted to their element as closely as Fishes, and perish almost, if not quite, as soon 

 when cast ashore, whilst the Ichthyosaurs were less limited in regard to medium, and had 

 a power upon dry land which neither of the other aquatic vertebrates enjoy. 



That our Sea-lizards occasionally sought the shore is to be inferred from the strong 

 inverted osseous arch supporting their fore fins, spanning across the chest from one 

 shoulder-joint to the other. In structure this arch closely resembles that in a group of 

 aquatic Mammals {Ornithorhpichus), which similarly surpass Cetacea in having a 

 command of both land and water, although, by their low position in the mammalian 

 class, they have closer alliance to the Reptilia. 



There is reason to infer, from examples of diminutive Ichthyosaurs fossilized within the 

 abdominal cage of larger ones, and with the snout directed toward, or partly protruding 

 from, the pelvic outlet, that they were ovo-viviparous and, as a rule, uniparous, reptiles.' 

 Others may have sought the shore for sleep or copulation, and have been enabled, by reaction 

 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 (Tab. XXIV, 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,' 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 



* " Note on the Dislocation of the Tail at a certain point ohservable in the Skeleton of many 

 Ichthyosauri," 'Trans, of the Geological Society,' 2nd series, vol. v, 1838, p. 511, pi. xlii. 



2 As first shown in the specimen of Ichthyosauri from the Lias of Boll, described and figured by 

 George Frieurich Jager, op. cit., tab. i, fig. 4 ; subsequently noted by Quenstedt in specimens in 

 the Tubingen Museum ; also by Channing Pearce (' Report of the British Association,' 1874). 



' " 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. 



