18 ('A]JFOi;>-IA ACADEMY OF 8('lKiN'CES 



tlic lione. The enamel is oniamented witli a lari;e number of line wrinkles 

 radiating' from the apex of the erown. 



Pterygoid Dentition. — Closely set teeth were present in at least five rows on the 

 heavy median plate of the pterygoid. They are of slender conical form and each 

 is set in a distinct pit of considerable depth. Unfortunately the crowns have 

 not been well preserved. They are all nearly circuhir in cross-section and the 

 enamel seems to have been slightly wrinkled toward tiie base. 



Finirtion. — The dentition of the Thalattosaurs is evidently tliat of swiftly 

 swimming fi>rms feeding in a large measure upon fish. The character of the 

 posterior mandibular and vomerine teeth of Tli<il(iif(i.'<(iiir>i><, indicating a crushing 

 function, probably means that this type fed also in })art u}>on some creature 

 with a shell which could be easily crushed. Such forms Avould have been found 

 in the numberless cephalopods of this epoch, and they doubtless furnished a 

 portion of the food of the Thalattosaurs. 



A'ertcbrae. 



ri.ATlO VII, FlOS. l-."!. 



In the type specimen of ThalaUosaurus alexaudrae parts of over thirty verte- 

 brae are shown. They represent the cervical, dorsal and anterior caudal regions. 

 A number of loose vertebrae in a fair state of preservation are associated with 

 other specimens. 



In Tliahiftosanrvs the centra of the anterior dorsals (PL vii, figs. 1 and 2) are 

 nearly circular in vertical, transverse cross-section. The anterior and posterior 

 faces are concave but not so deeply excavated as in the Ichthyosauria, the wall 

 betwet'n the two faces representing more than half of the antero-posterior diam- 

 eter of the centrum. The upper arches are rather slender and are not greatly 

 thickened. Tlie anterior margins are sharp. The posterior edges seem to be 

 blunted or rounded. Strong zygapophyses are present. A characteristic feature 

 of these vertebrae seems to be found in the closer attachment of the upper arches 

 to the centra than we find in the Ichthyosauria. In this respect they resemble 

 the vertebrae of the ]*lesiosaurs and (-rocodiles. 



An upper arch of T. shastensis (PI. vii, tig. 4) shows the structure of the 

 neurocentra better than in any other specimen. The spine is wider antero- 

 posteriorly and is not so high as in T. akxandrae. Possibly this is due in this 

 case to the arch having occupied a more anterior position than the arch of 

 alexandfrae figured (PL vii, fig. 1). The zygapophyses are large and strong and 

 the facets well separated. 



Tlie ribs of TJialaffosaitrus articulate on a single wide apoj)liysis, the upper 

 port ion (diapophysial) of which is situated above tlie middle of tlie ceiiti'um. 



