Mi;i!i;iA>[ THK THALATTDSATItIA 29 



the Thalattosaurs bv the more skMuk'r snout, the terminal jiosition of tlie nares 

 and tlie corresjjondintilv different structure of tlie rostral re,s;ion, the absence of 

 a j)ineal foi-anicn and of a coronoid process, the robustness of the ribs and the 

 bicipital ai'tieidation of those in the anterior dorsal region, tbe different form of 

 the elements in botb pectoral and peh'ic arches, and the rclativ<'ly but little 

 specialized limbs. 



In the lenjfthening of the snout tbe ( 'iioristodera are more specialized than 

 the Thalattosaurs. l)Ut the ternnnal position of the external nares shows the kind 

 of specialization which we hnd mainly in aquatic reptiles belonging in fresh 

 water, while the position of the nares in the Thalattosaurs is the form of special- 

 ization seen generally in marine types. 



Wlien we consider along with other things that tlic ( "Iioristodera are in some 

 respects much less specialized than the Thalattosaurs, and that they occur two 

 geological periods later, it is evident that they are not closely related. 



Reseviblaiice to Proterusuchus. — Some interesting resemblances to the Thalatto- 

 saurs are shown in Pmternsxchvs recently described l)y Broom. ^ This important 

 form is known oidv from the anterior portion of a skull reciMitly discovereil in 

 the Karoo beds of Tarkastad, South Africa. In it we find dentigerous prevomers 

 and pterygoids with edentulous palatines. The prevomers reach back to meet 

 the pterygoids broadly. On the upper side of the pterygoids are high, thin 

 vertical plates. The characters of this form am[)ly justify its determination by 

 Broom as "a primitive Rhynchocephalian which shows a considerable degree of 

 specialization along a line which gave rise to the crocodiles. . .'" 



The cond)ination of characters found in Prdfcrdsuclnis is in som(> respects 

 similar to that seen in Th((laftosaurus, and as is shown i)y i>room for Proterosiirlnix, 

 the palatine region exhibits a general resend)lance to that of Pnivolojilioti. In 

 other characters, as in the presence of large antorbital vacuities, the anterior 

 position of the external nares, the position of the internal nares farther forward 

 and separated by the main bodies of the prevomers, in the absence of a coronoid 

 process, and in the large size and hcavv fangs of the slightly differentiated teeth, 

 Proferosuclius shows itself to be quite different from Thalattosaurus. The resem- 

 blance exhibited by these forms is evidently in the main due to their approxima- 

 tion to the type of the primitive Rynchocephalia. Such similarities as we tind 

 are not sufficient to permit our including them both in the same order. In 

 adaptation to acjuatic con<litions Proterosiiflnis might have led to some such foi'ms 

 as the Parasuehians. while the Thalattosaurs stand as a type very different from 

 this order. 



ReJationxhi p to the Protoro^anr'nt. — N'ery close aflftnity with any of the forms 



1 R. Broom, .\nn. South .\frican Mus. v. 4, art. 7, p. 159, pi. xix. Nov. 1903. 



