220 EXTINCT ORDEES OF REPTILES. 



ceptionally large, and though the nostrils are placed close 

 beside the orbits, as in Ichthyosaurus, there is no sclerotic 

 ring. The teeth are conical and pointed, with longitudinally 

 striated crowns, but each is sunk in an independent socket. 

 The vertebral centra are only slightly biconcave, and the 

 neutral arches are united with them by osseous junction. 

 The pectoral arch consists of a large coracoid and scapula on 

 each side, but clavicles and interclavicle may be apparently 

 wanting. The ]3addles are proportionately longer than those 

 of Ichthyosaurus, and though formed on the same plan, consist 

 only of the five digits, without marginal ossicles in addition. 

 The pelvic arch is well developed, and there is a sacrum of 

 two vertebne. As regards the habits of the Plcsiosaurus, Dr 

 Conybeare arrives at the following conclusions : " That it 

 was aquatic is evident from the form of its paddles ; tliat 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 extremities 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 j)rogress 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 

 liave 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 per- 

 haps 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 

 tlie 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 Plcsiosaurtis is from the Lias 

 to the Chalk inclusive, and specimens have been found in- 

 dicating a length of from eighteen to twenty feet. About 

 twenty species of Plcsiosaurus have been described in all. 



