28 OALIFOKNIA ACADEMY OF SCIKXCKS 



again in the Mosasaurs. This specialization would at the same time take tlic 

 Thalattosaurs out of the direct line of I'volution. It is to be noticed in this 

 connection that the smallest and least specialized form, seen in Nectosaurus, is 

 nearer the Lacertilia than the large]- and more specialized Thalattosaurus. 



In PaligiuDia Broom, of the South African Triassic, we have represented an 

 undoubted Lacertilian with the lower temporal arch absent and only a minute 

 process projecting from the posterior side of the jugal. Tlie ([uadrate is typically 

 lacertilian as are also the relations of the elements in the upper temporal bar. 

 This form shows that the characteristics of the Squamata were expressed very 

 early and makes more probalile the discovery of lacertilian tendencies in early 

 diaptosaurian groups. 



Relationshi-p to the Proganosauria. — As possible close relatives of the Thalat- 

 tosaurs, the Proganosauria claim especial notice,- being a group of ratlicr prim- 

 itive aquatic forms having affinities with the Rhynchocephalia and occurring 

 earlier in geological time than the Thalattosaurs, and therefore possibly ancestral 

 to them. Ihifcn-tunately the part of the thalattosaurian skeleton of which we 

 have the best knowledge, viz. the skull, is the most imperfectly known part in tlie 

 Proganosauria. Osborn considers the skull probably double-barred, though the 

 structure of the temporal region is as yet unknown. The position of the superior 

 nares and the .structure of the palate are likewise unknown. Teeth are present 

 on the roof of the mouth on what are supposed to be the palatines, while in the 

 Thalattosaurs they are absent from the palatines, and present on the prevomer 

 and pterygoids. The skull is considerably longer and more slender in tlie Pro- 

 ganosauria and the long, slender teeth are more numerous. 



In the structure of the better known parts of the sktdeton of the Progan- 

 osaurs the separation is wide. The small centra and greatly expanded upper 

 arches of the vertebrae and tlie peculiar narrow-headed ribs of the Proganosaurs 

 stand in strong contrast to the relatively small upper arches and the broad- 

 headed ribs of the Thalattosaurs. In the pectoral girdles of the two there is 

 some similarity excepting in the scapula. The plate-like pelvis of the Progan- 

 osaurs is much more primitive than that of the Thalattosaurs and the limlis of 

 the former have not gone nearly so far in specialization, particularly in the 

 shortening and broadening of the epipodial elements. 



Relationship to the Choristodera. — The aquatic diaptosaurian forms included 

 in the Choristodera resemble the Thalattosaurs mainly in that they are acjuatic 

 and have rhynchocephalian affinities. Here, as in the Thalattosaurg, there is a 

 vomerine dentition l)ut the teeth are small, very numer(nis, of an entirely differ- 

 ent form, and are set in several rows. The pterygoitl dentition is also different 

 and the palatines bear teeth. The Choristodera are farther distinguished from 



