MERRIAM THE THALATTOISAX^RIA 31 



Rclatioiiiiliip to titc Iilnjiiclioccphaliu. — lu a large number of the skeletal 

 characters, strong similarities to the Thalattosaurs are found in various mem- 

 bers of the Khynchocephalia. In the general plan of the skull there are many 

 points of resemblance, and the uniformity of this resemblance throughout the 

 various parts is particularly noticeable. In some respects we find in 

 the Rhynehocephalia approximations to thalattosaurian characters not seen 

 elsewhere. 



In the superior temporal region the similarity of Sauranodon and Pleuro- 

 saurits to the Thalattosaurs is evident. The parietals here extend around the 

 anterior ends of the upper fenestrae as in Belodon, and in both of these genera 

 thej' meet the squamosals on the outer side of the upper temporal openings. 



In the structure of the arches and ribs many common characters are found, 

 although the vertebrae are not closely similar. The similarity of the more 

 slender pelvic elements is particularlv noticeable, being closer than betAveen the 

 thalattosaurian elements and the primitive plate-like pelvis of the Protorosauria 

 and Proganosauria or the liroad pelvic elements of the Choristodera. 



As discriminating characters, we find that no true Rhynchocephalian pos- 

 sesses a vomerine or pterygoid dentition or shows the degree of adaptation of the 

 limbs and skull to aquatic conditions that we find in the Thalattosaurs. No 

 member of this order has such highly specialized limbs and in none do we find 

 the thalattosaurian structure of the rostral region. Pleurosaurus and Acrosaurus 

 were aquatic forms, but the limbs were only slightly modified. The structure of 

 the nasal region in Pleurosanrus approaches that of the Thalattosaurs but does 

 not appear to be the same. Moreover, in Pleurosaurns where we find some of the 

 closest i'e.semblances, the structure of the lateral temporal region appears very 

 different. From Lortet's figures^ it would seem that there is no lateral temporal 

 vacuity here. Such was also the impression obtained by the writer in a hasty 

 examination of his specimen. 



As all of the Rhynehocephalia which we have for comparison are much 

 younger than the Thalattosaurs, an<l are all in many respects less specialized, it 

 is useless to atten;pt to place them in any ver^- close relationship to the Thalatto- 

 sauria. It is probable that they are derived from a persistent land or shore type 

 from which the Thalattosaurs are an early offshoot. 



Conclusions. 



I'^roni the comparisons that have been made, it is evident that the Thalatto- 

 saurs are to be considered as an independent group. The orders with which 



1 Kept. Foss. du Bivs'sin du Rhone, 18!M. PI. viii, tigs. 1 and 2. 



