li) CALIKOIJMA ACADIOIV OK S<TKN('KS 



;i coiisidcraltlc part of tlic iiiiicr side of the jaw. It oxtondis bark to a point below 

 tlic middle of the coronoid and may reach forward almost, if not quite, to the 

 symphysis. 



The most characteristic feature of the mandible is the form of the coronoid. 

 This (dement is broadly expanded over the inner side of the ramus and reaches 

 with its upper border high above the supra-angular as a coronoid projection. 

 The form of tlie coi'onoid is strongly suggestive of the Lacertilia. though it is 

 approached in the true lihynchocephalia. Particulai'ly is tliis noticeable in 

 the jaw of Necf(imi(rH<!, wliicli taken tiy itself would appear unquestionably 

 lacertilian. 



I>ciitition. 



The dentition is well shown in the tyite specimens of ThaJattosavrus, as also 

 in the lower jaw of the type of Necfomnrun, and more or less imperfectly in most 

 of the other specimens. In addition to the jaw elements, the pterygoid and 

 prevouier are (U'litigerous while the palatines are not known to have carried teeth. 



Teetli oftheJav's. — The liisrrtioit of the teeth on the ui)])er and lower jaws 

 varies considerably within the group. In ThalnHdmurus the teeth of both upper 

 and lower jaws seem to rest in pits, which may be in elevated bases formed by 

 the alveolar margin. In some cases they appear to be fused Avith the jaw bone. 

 In T. perritii the teeth of both jaws rest in distinct pits. The depth of the pit 

 and the general character of the insertion are to a considerable extent dependent 

 on the form of the crown. The slender conical, anterior teeth are very deej)ly 

 set in the jaws, while the pits for the low, broad-crowned, posterior mandibular 

 teeth are much shallower. This may have been the case in T. xlexaudrae also, 

 but the preservation of the anterior portion of tlu' jaw (jf the type specimen has 

 not ])ermitted a satisfactory investigation of the insertion. 



One of the diagnostic features of Thahittosaurm is found in the remarkable 

 differentiatiDU of the mandil)ular <lentition. In T. alexandrae (IM. i, tig. 3) the 

 crowns of the anterior teeth are slender conical with a slight lateral compression, 

 while the most posterior teeth are very broad and nearly Hat-crowned, resembling 

 considerably the form of the teeth of Phtcndns, although very much smaller. 

 The teeth in the middle portion of the jaAv grade from the Hat form to slender 

 conical, through the gradual development of a tubercle on the antero-external 

 portion of the croAvn. This elevation increases in size until it comprises the 

 whole crown. Unfortunately the dentition of the upper jaws is not well known 

 except on the anterior portion, where the crowns are conical as on the 

 opposing portion of the mandible. Ujxm the maxillary, portions of two or three 

 fangs are seen, but the crowns have lieen destroyed. 



