MKIv'KIAM TllK THALATTOSAl'IUA 13 



'r\w pirfniiita/s arc lari;c robust elements extending t'nini the middle of the 

 upper boundary of the orl)it forward along tlu' inner side of the maxillary. 



The small hii'linjiiKtl, extending over only a limited area of the face, is well 

 shown in the tvi)e of 'riidlnttomnrus (PL i, tig. 1, L). The laehrynuil foramen is 

 also distinctly seen here. In cross-section /), Plate n, ligure 16, .1 there is seen a 

 peculiar element apparently distinct from the nasals, maxillaries, premaxillaries 

 and prt'vomer. If suidi is I'eally the ease, this is prohahly an anterior j)rolongation 

 of the })refrontal. This would, however, rec^uire an extraordinary enlargement 

 of the anterior end of this hone, and further evidence is i-e(|uired before its 

 relations can be satisfactorily (letermine(l. 



Palatine Region. — It is in the palatine region that some of the most dis- 

 tinctive characters are seen in the nieiidiers of this grou[i. The general aspect 

 here is of a primitive form in which the rostral region, and in it pai'ticularly the 

 vomerine dentition, exhibits distinctive specialization. 



The position and relations of the jiretWHer are iiest shown in the type of 

 Thalattosaurus (Tl. i. tig. 2 and PI. ii, tigs. V> and Ir). It is here seen to lie 

 largely between the maxillaries although reaching far foi'ward along the rostrum. 

 It is much elongated antero-posteriorly and has in general a sphenoidal form. 

 So far as can be determined from the sections no median suture is present, 

 though the structure niav have been })aired. From the meilian portion of the 

 posterior end three laniiinie project backward, the meilian plate being much 

 smaller than the others. ()u either side of these laminae the posterior margin 

 slimvs a regularly roun(le<l excavation, representing the anterior borders of the 

 inferior nasal openings. The posterior plates evidently connected the prevomer 

 with the palatines or the pterygoids. The palatal surface of the prevomer 

 bears two long rows of teeth following the lateral margins. Posteriorly they 

 are separated by quite a distance, but they approach each other near the middle 

 of the l)one and run paralhd from that point forward. 



In another specimen ( I'l. v, Hgs. '2(t and 21)), which was found separated from 

 other skeletal remains, exactly the same foi'm is shown. In the type of T. perrini 

 ( IM. V, tig. o) a part of a prevomer of the same general form is known. Fragments 

 of several other sjiecimens have been found to show corresjionding structure. On 

 all the specimens in whi(di the antcrioi' end of the prevomer is shown, the dentig- 

 erous portion of the bone is seen to bend downward sbari)ly in the middle. 

 In the specimen which was entirely separated from other bones of the cranium 

 (PL V, figs. 2a and 26), the drop is very pronounced and cannot be fully accounted 

 for by distortion. A similar but less prominent lieiid is set'u in the prevomer 

 of 7'. iierriiii. Evidentl\- the middle poi-tioii of the prevomer was produced 

 downward somewhat, making the tooth row convex. The downward sweep is, 



