10 



<'AT>IK(>i;XlA ACADK.MV OF S('IF,.\( 'KS 



Tlie iKisals are clearly shown in the type of Thdldftnsdtinis. They are not 

 large and horder mainly the median and posterior sides of the superior mires. 

 They do not meet medially but are separated by the frontals and premaxillaries. 

 The posterior ends rest between the forks of the frontals. A tendency toward 

 separation of the nasals by extension of the frontals and premaxillarit's is seen 

 in many reptilian skulls, particularly in rhynchocephalian, lacertilian and sauro- 

 pterygian forms, but such t'omplete sepai'ation as is shown here is so rare that 

 it may be considered an important character. The neai'est approach to this 

 structure is seen in the Sauropterygia. 



In that group the nares are situated close to the orbits and are either 

 median or sul)tei'minal. in all cases the premaxillaries foi-m a large share of 

 the snout. In the Nothosaurs the nasals have almost the same form as in 

 Thalattosmiriis, and the premaxillaries are thrust back between the nares though 

 not in contact with the frontals. In Pisfd.^aiiru.'^ the premaxillaries touch the 

 frontals, separating the nares and the small nasals. In Plesiosaurns the pre- 

 maxillaries ai'e in broad contact with the frontals. in /hilichorln/iichojis, vavvntly 

 described by VVilliston. the premaxillaries extend hack to the parietals, sepa- 

 rating even the frontals. 



Fig. '2. TlialdlldsadriiK iiU:iiui(lriii'. Kci'inistnictcil sUull, sn|i('rii>v view, X ^r. /')/(. |ireiiiaxillary ; 

 .1/, maxillary; .V, nasal; ]C, .«uinTiiir nares; /', t'rcnital; /'/■, i>ariftal; I'li. iiincal fnranii'ii: /'/', iirt'fnintal ; 

 P(o, liost-friintd-iirbital ; ./, jnijal; 7^, laclii'vnial ; /'/, jialatine ; N(/, si|nani(isal ; 't'. i|uaili'atr. 



Froiito-parietuI Region. — This region is particularly well shown in the speci- 

 men of 'riidUtttnmnrus represented in Plate iii. Both frontals and parietals are 

 distinctly i)aii'e(l and on both the superior surface is nearly smooth, being 

 marked only by Hne lines and pits. The /Ve;/^//.s- are relatively large. Their 

 postero-external angles reach far back and partly clasp the anterior ends of the 

 parietals. Tlu' anterior ends are deeply incised for the reception of the nasals. 

 The median anterior forks extend forward as far as the middle of the superior 

 nares, while the lateral forks reach almost if not (piite to the outer border of 

 the nares. 



