394 S. W. WILLISTON 



The elongation of the cervical vertebrae was clearly a specializa- 

 tion and not an ancestral character; but aside from this and the 

 peculiar scapula, I can see nothing in the entire skeleton that 

 might not have been confidently predicted in the ancestral lizard. 

 It may be objected that the thecodont, or more properly proto- 

 thecodont, teeth were an aberrant character; that the lizard 

 ancestor must have had, like the living ones, acrodont or pleurodont 

 teeth. On the contrary, I believe that the teeth primitively in 

 the Squamata were protothecodont, which, by the loss of the 

 parapet of bone on the inner side of the maxillae, and the anterior 

 part of the coronoid, became pleurodont. Moreover, the teeth 

 in the mosasaurs are attached in shallow cavities. In Fig. 3, G, 

 I give a sketch drawing of the skull of Opetiosaurus Kramb., the 

 earliest known skull of a squama te reptile, from the uppermost 

 part of the Lower Cretaceous. The figure was made by me from 

 the type specimen in the Munich museum, and differs somewhat 

 from previous ones by Kramberger and Nopsca. 



Proterosaurus. — There are certain marked resemblances between 

 Araeoscelis and Protorosaurus which may, and probably do, indi- 

 cate genetic relationships. Unfortunately much doubt remains 

 as to the structure of the skull in this genus, notwithstanding the 

 numerous specimens which have been studied during the past two 

 centuries. Seeley's interpretation of the skull structure of P. 

 speneri has been received with considerable doubt by later students. 

 I have examined the specimen on which he based his results chiefly, 

 preserved in the collection of the Royal College of Surgeons, but 

 could make little of it; my examination, however, was brief, and 

 without the aid of Seeley's paper. Nevertheless, I am inclined 

 to the belief that his interpretation of the elements will eventually 

 be found to be approximately correct. He figures the temporal 

 region as open at the sides of the narrow parietals. If it should 

 be found that the side was largely covered below by the squamosal, 

 leaving a vacuity above, the structure would be essentially like 

 that of Araeoscelis. 



There is certainly as much justification for this hypothesis as 

 there is for the one usually accepted, the presence of two temporal 

 arches, for which I can see no evidence whatever. Nevertheless, 



