250 ME. C. W. ANDREWS ON THE STRUCTURE [May 1 896, 



of Sphenodon. Koken * has expressed the same opinion as to the 

 constitution of the ' squamosal ' in the jNothosauria. In several 

 Plesiosaurian skulls in the British Museum the suture between 

 these elements is distinct. 



The quadrate (q.) is a long, stout bone ; posteriorly it is convex 

 from side to side, anteriorly concave. It projects downward and 

 backward, and the condyle for the mandible lies somewhat below 

 the level of the alveolar border of the maxilla. On its outer side 

 the inferior ramus of the squamosal is closely adherent to it, and 

 extends nearly down to the condyle. 



In Cimoliosaurus Cope 2 has figured a small quadrato-jugal, and 

 Koken 3 has recorded the probable occurrence of this bone in Notho- 

 saurus ; it therefore seems possible that the Plesiosaurian quadrate 

 may be a fusion of the quadrate and quadrato-jugal, a view which 

 derives some support from the fact that the relations of the 

 squamosal to the ' quadrate ' are almost exactly similar to those 

 existing between the squamosal and the quadrato-jugal in Splienodon. 



The general structure of the upper surface of the skull is shown 

 in PL IX. fig. 2. It will be seen that between the anterior halves 

 of the temporal fossae the parietals form a high, sharp crest, but that 

 posteriorly they widen out into a broad triangular plate, convex 

 from side to side, which apparently roofs in the brain-case. The 

 outer angles of this plate are overlapped by the upper rami of the 

 squamosals, these forming the hinder border of the temporal fossae. 

 In front, opposite the anterior end of these fossae, the parietals en- 

 close the pineal foramen, which does not extend into the frontals, 

 and laterally they widen out and take part in the formation of the 

 postorbital wall. There is clearly a distinct post-temporal fossa, 

 closed above by the lateral process of the parietal and the upper 

 ramus of the squamosal. The frontals extend much farther forward 

 than in Peloneustes, and separate the external nares. I can find no 

 clear evidence of the existence of distinct nasals and lachrymals. 



Comparison of the palatal portion of this skull with that of Pelo- 

 nenstes shows that the chief difference between them is that in the 

 latter the pterygoids, instead of merely articulating with the sides 

 of the basisphenoid, overlap it, and form a median suture with one 

 another on its ventral surface. In Peloneustes, also, the form of the 

 parasphenoid is different, and it is very uncertain whether there is 

 any suborbital vacuity. 



In Nothosaurus the pterygoids meet in the middle line from end 

 to end, and there is no suborbital vacuity, so that the palate is 

 completely closed ; this appears to be a more specialized condition 

 than occurs in either Plesiosaurus or Peloneustes, although both 

 these genera are of a later date. 



1 ' Beitrage zur Kentniss der Gattung Nothosaurus,' Zeitschr. deutsch. geol. 

 Gesellsch. vol. xlv. (1893) p. 363. 



2 ' On the Structure of* the Skull in the Plesiosaurian Reptilia,' Proc. Auier. 

 Phil. Soc. Philadelphia, vol. xxxiii. (1894) p. 110. 



3 Op. supra cit. 



