ADAPTATIONS 105 



that some of the later and more specialised forms were without 

 teeth. Whether, however, they developed in compensation any 

 structure comparable to whalebone, or baleen, we shall never 

 know. 



Plesiosaurs (order Sauropterygia) were very different-look- 

 ing reptiles to ichthyosaurs externally, from which they also 

 display marked divergence in the structure of the skeleton, in- 

 dicating their derivation from a totally distinct ancestral stock. 

 In the more typical forms, such as Plesiosaurus and Thauma- 

 tosaurus of the Lias, the body was somewhat turtle-shaped, the 

 tail comparatively short and pointed, the neck very long and 

 slender, the head relatively small, and both pairs of paddles 

 long and narrow. Apparently there was no fin on either the 

 back or the tail, and the eyes lacked a bony sclerotic ring. 

 Possibly the latter feature indicates that the plesiosaurs were 

 not in the habit of descending to such depths as some of the 

 ichthyosaurs — a suggestion supported by the shape of the 

 former, which does not seem to be one well adapted for 

 diving. Not improbably they swam with the whole neck 

 and body partially above the surface and frequented the neigh- 

 bourhood of the shore. Owing to the shortness of the tail and 

 the length of the paddles, their mode of swimming must have 

 been quite different to that of the ichthyosaurs, the paddles 

 being the sole propelling instruments. Some of the species 

 equalled the largest ichthyosaurs in point of size. Much the 

 same contour of head, neck, and body is displayed by the ple- 

 siosaurs of the Middle and Upper Jurassic and Cretaceous 

 strata, such as Murcenosaurus and Ciwoliosaurus, although in 

 some cases the neck was relatively longer than in their Liassic 

 predecessors. On the other hand, the gigantic pliosaurs (P/w- 

 saurus) of the Oxford and Kimeridge Clays developed an 

 immense and ponderous head, which necessarily entailed for its 

 support a short and thick neck. These are more adapted for a 

 pelagic life, although not to the same degree as the whale-like 

 Ophthalmosaiirus among the ichthyosaurs. The earliest reptiles 

 that have been referred to the Sauropterygia are small Triassic 

 species (Neusticosaurus) of only about a foot in length, in 

 which the limbs appear to have formed less perfect paddles, 

 and whose habits were probably subaquatic and fresh-water. 

 With the close of the Cretaceous epoch, the plesiosaurs, like 



