166 



EEPTILIA 



CLASS III 



The members of this order differ conspicuously from all living reptiles, 

 and are distinguished chiefly by their fish-like form of body, paddle-shaped 

 limbs with numerous oval or polygonal phalanges, large head with elongated 

 rostrum, short amphicoelous vertebrae, and naked integument. The snout 

 resembles that of a dolphin, the teeth are crocodilian-like, the skull and 

 pectoral girdle suggest those of lizards, the limbs are very similar to the 

 flippers of marine mammals, and the vertebrae and caudal fin are decidedly 

 fish-like. As regards external form, limb-structure, and adaptation to a marine 

 existence, they depart as widely from other reptiles as whales do from land 

 mammals,. and occiipy as isolated a position. 



The absence of branchial arches and shape of the hyoid bones prove that 

 respiration was pulmonary among Ichthyosaurs ; and their viviparous habit is 

 demonstrated by a number of well-preserved skeletons in the Lias of England 

 and Würtemberg, which exhibit as many as seven embryonic individuals in 

 the abdominal cavity. Large quantities of their coprolites occur in various 

 localities, and contain the scales and bones of fishes, and fragments of cephalo- 

 pods. Ichthvosaurian remains are found exclusively in the marine deposits of 

 the Mesozoic, and are most abundant in the Lias. Of their origin nothing is 

 known, although it is certain that they were derived from land animals. The 

 earlier (Triassic) forms have less completely paddle-shaped extremities than 

 the later. Some species attain a length of 9 m., and the smallest are only 

 about 1 m. long. 



The skull (Fig. 262) is conspicuous for its elongated, dolphin-like rostrum 

 and enormous orbits, which are surrounded by a ring of fifteen to nineteen scler- 



Ichthyosaurus acutirostris, Owen. Upper Lias ; Curcy, Calvados. Skull, supcrior aiul lateral aspects, V-i- 

 A, Orbit ; ang, Articular ; d, Dentary ; fr, Frontal ; j, Jugal ; Je, Surangular ; md, Mandible ; mx, Maxilla ; A, 

 External narial opening ; na, Nasal ; op, Splenial ; pa, Parietal ; 2Wix, Premaxilla ; por, Postorbital ; ptf, Post- 

 frontal ; Qvj, Quadrato-jugal ; S, Supratemporal vacuity ; sei, Sclerotic plates ; si/, S(iuainosal ; st, Supratcmporal 

 or prosquamosal (after E. Deslongchamps). 



otic plates. The paired parietals and frontals are of small size, and a large 

 parietal foramen occurs at their junction in the median line. Adjoining the 

 parietals on either side are the large oval supratemporal vacuities (S), bounded 



