41 8 MANUAL OF ZOOLOGY. 



The limbs agree with those of the Ichthyosauri in being in the 

 form of swimming-paddles (fig. 164), but differ in not possessing 

 any supernumerary marginal ossicles. A pectoral arch, formed 

 of two clavicles and an iriterclavicle (episternum) appears to 

 have been sometimes, if not always, present. The teeth were 

 simple, and were inserted into distinct sockets, and not lodged 

 in a common groove. 





Fig. 164. Plesiosaurus dolichodeirus. 



The most familiar and typical member of the Sauropterygia 

 is the Plesiosaurus (fig. 164), a gigantic marine reptile, chiefly 

 characteristic of the Lias and Oolites. As regards the habits 

 of the Plesiosaurus, Dr Conybeare arrives at the following con- 

 clusions : " That it was aquatic is evident from the form of 

 its paddles ; that it was marine is almost equally so from the 

 remains with which it is universally associated; that it may 

 have occasionally visited the shore, the resemblance of its ex- 

 tremities to those of the Turtles may lead us to conjecture; its 

 movements, however, must have been very awkward on land; 

 and its long neck must have impeded its progress through the 

 water, presenting a striking contrast to the organisation which 

 so admirably fits the Ichthyosaurus to cut through the waves." 

 As its respiratory organs were such that it must of necessity 

 have required to obtain air frequently, we may conclude " that 

 it swam upon or near the surface, arching back its long neck 

 like a swan, and occasionally darting it down at the fish which 

 happened to float within its reach. It may, perhaps, have 

 lurked in shoal water along the coast, concealed amongst the 

 sea-weed ; and raising its nostrils to a level with the surface 

 from a considerable depth, may have found a secure retreat 

 from the assaults of powerful enemies ; while the length and 

 flexibility of its neck may have compensated for the want of 

 strength in its jaws, and its incapacity for swift motion through 

 the water." 



The geological range of the Plesiosaurus is from the Lias to 

 the Chalk inclusive, and specimens have been found indicating 

 a length of from eighteen to twenty feet. 



Of the other genera of the Sauropterygia, Simosaurus and 

 Nothosaurus are from the Trias, and are chiefly characteristic 



