30 CALIFOKNIA ACADEMY OF SCIENCES 



ill tliis order is precluded by the totally different structure and function of the 

 limbs, the character of the i)elvis, and to a considerable extent also of the lower 

 jaw and the facial region. 



The skull of Palaeohatteria differs from tliat of the Thalattosaurs in tlie 

 terminal position of the superior nares, the small and short premaxillaries, 

 large maxillaries and lachrymals, very large nasals, separate postfrontal and 

 postorbital, absence of coronoid or at least of a prominent coronoid projection, 

 and in the })resence of large prehensile teeth on the posterior [)ortion of the jaws. 

 It might be presumed that the greater number of these differences could be 

 brought about by retrogression of the narial openings with corresponding 

 enlargement of the premaxillaries, in the evolution of an aquatic form from a 

 type originally like Palaeohatteria. There are, however, few real similarities con- 

 necting -P«/aeo/t«^<er/o and the Thalattosaurs which would not at the same time 

 show affinities Avith the majority of the older diaj)sidan orders. While such 

 modification may be considered possible, there is nothing to show that it has 

 actually occurred. 



The relationship to Protorosaurns is hardly nearer than to I'dhicdhatfcr/a. 

 The limbs, girdles and vertebrae are very different, and the skull is not more 

 similar. The facial region in some respects seems to present a stronger resemblance 

 than to PiiaeoJtattcria and if, as Seeley has suggested, the openings immediately 

 in front of the orbits in Protorosaurus are really the superior nares, there would 

 be an important feature common to the two. The nasals are, however, very 

 large, reaching far forward as in the Ichthyosaurs, and the premaxillaries appear 

 to have no posterior stem. In the palatine region we find the prevomer long 

 and slender, and the palatines swinging around the narial openings and coming 

 into broad contact with the prevomers in front of the nares as well as behind 

 them. 



Evidently no close affinities are to be found with the Protorosauria, and 

 while we must suppose the Thalattosaurs to be derived from Permian land or 

 shore forms, it would not be possible to unite them with this group or even to 

 consider the Protorosauria as the ancestral type. As has been shown by Seeley, 

 Nopcsa, (_)sborn and others the trend of evolution in the Protorosauria was 

 already decidedly toward the development of specialized land forms, as seen in 

 the Dinosauria. 



The Thalattosaurs are probably derived from land forms, as is evidenced 

 b}' the character of the pelvis and of the neural spines of the vertebrae. The 

 Protorosaurs probably come nearer than any other group to this primitive type, 

 but are not themselves the ancestral forms which we seek. 



