26 CALIFORNIA ACADEMY OF SCIENCES 



veiy different types out of tlie same stock. The Iclitlu'osaurs may have taken 

 to the sea earher, and the Tlialattosaurs have originated from a later and there- 

 fore somewhat different pliase of the same group. On tlie otlier liand the 

 development of such differences as we see here must have required much time, 

 and it may well he questioned whether the time of origin of the lehthyosaux's 

 may not date back so far that it would be straining a point to call its ancestors 

 Diaptosaurian or even Diapsidan. 



The remaining situation, in wliicli we would have the two groups converging 

 from fundamentally different stocks, seems in many respects the most natural 

 relation. At any rate, the two groups must be considered as widely separated, 

 and neither may be judged to be ancestral to the other. If they are not closely 

 related ami it should appear that of the two the Tlialattosaurs are nearer to the 

 Rhynchocephalia, we ma}- perhaps reasonably (juestion the rhynchocephalian or 

 diaptosaurian origin of the Ichthyosaurs. 



ParasKcJilcni Characters. — The general skull structure of the Tlialattosaurs has 

 a certain degree of resemblance to tliat in Belodon, particularly in the rostral and 

 temporal regions. The position of the nares is similar, as also tlie general form of 

 the premaxillaries and maxillaries, though the premaxillaries do not separate the 

 nasals and reach the frontals. The superior temporal openings are similarly 

 situated low down between the upper temporal bars, and the parietals bound 

 the anterior side to a greater extent than is usual. There is also some similarity 

 in the structure of the palate, and the gnathic dentition of Behxlon is not unlike 

 that of Thakittnsanrus pc^rini. Taken as a whole, however, the two skull types 

 are very dissimilar. The superior nares of Belodon are in the large nasals; the 

 premaxillaries do not reach hack to the frontals; large antorl)ital vacuities are 

 present; the lachrymals are large, while the prefrontals, frontals and parietals 

 are relatively very small; the postorbital and postfrontal are separate; tliere is 

 no pineal foramen; the form of the quadrate is very different from that of Tlta- 

 lattosavnis; the palatine borders the inferior nares externally; the coronoid is 

 small and inferiorly situated; there are no teeth on the prevomers or pterygoids; 

 and there is a large mandibular vacuity. These differences together with the total 

 dissimilarity in the structure of the vertebrae, ribs and lindjs show that the two 

 groups are ordinally distinct. The general outlines of the structure in both arc 

 those of the primitive diapsidan Reptilia. Both are aquatic types and to a slight 

 extent this has tended to bring them nearer together. The possibilities of 

 evolution in the marine Thalattosauria were, however, much greater and also 

 specifically different from those open to the fresh ivater Parasuchians, and the 

 result of specialization has been the production of very different forms. As 

 different as these two ai'e. it may he noted that they are at least as near together 

 as the Tlialattosaurs and the Ichthyosaurs, although the latter are marine. 



