252 MR. C. W. ANDREWS ON THE STRUCTURE [May 1 896, 



on its pterygoids and vomers is probably a primitive character 

 derived from its Labyrinthodont ancestors ; the palate of Pareia- 

 saurus is similar. 



In the Theriodonts a short secondary hard palate is developed, 

 carrying back the opening of the internal nares ; but in some speci- 

 mens (for example, the skull of Galesaurus planiceps, B.M. R. 511) 

 the relations of the bones constituting the primitive palate are 

 Sphenodon-like, the pterygoids extending forward to meet the 

 vomers, and their lateral rami bearing a downwardly-directed 

 process (ectopterygoid) which lies against the inner side of the 

 closed mandible, and is no doubt partly formed by a transpalatine 

 element. There seems to be no suborbital vacuity. 



The palatal structures of the Chelonia, regarded as modifications 

 of the same type, are easily comprehensible, and the same is the 

 case with the Lacertilia. In the highly specialized palate of the 

 Crocodilia, the resemblance to the primitive form is masked by the 

 secondary hard palate formed by the palatines and pterygoids : but if 

 this be disregarded, the same type of structure may be traced here also. 



Enough has been said to show that among reptiles a certain 

 similarity of palatal structure does not necessarily imply any close 

 relationship, but the very great resemblances existing between the 

 Plesiosaurian and Rhynchocephalian palates, reinforced by the 

 numerous other points of likeness in other portions of their skeletons 

 pointed out by Baur, lead to the conclusion that the Sauropterygia, 

 notwithstanding their single temporal arcade and thecodont denti- 

 tion, are descended from a primitive Rhynchocephalian reptile. This 

 opinion has already been expressed by several writers, notably by 

 Baur x and Boulenger. 2 



EXPLANATION OF PLATE IX. 



Skull of Plesiosaurus macrocephalus, Buckland. 



Fig. 1. From below. 



2. From abo^e. 



3. From the side. 



4. Temporal fossa seen obliquely from the side, showing the relations of 



the columella. 



ang., angular. 

 b.oc, basioccipital. 

 b.sph., basisphenoid. 

 col., columella cranii. 

 ext.nar., external nares. 

 int.nar., internal nares. 

 fr., frontal. 



jty-> jugal- 

 mx., maxilla. 

 orb., orbit. 

 pal., palatine. 



pas., parasphenoid. 

 pin.for., pineal foramen 



[misprinted pm.for.]. 

 pmx., premaxilla. 

 p.orb., post-orbital. 

 p.fr., pre-frontal. 

 ? post.fr., ? separate post-frontal. 

 post.pal.vac, posterior palatine 



vacuities. 

 q., quadrate, 

 squamosal. 



par., parietal. ' vom., vomer. 



All the figures are about £ natural size. 



1 ' On the Phylogenetic Arrangement of the Sauropsida,' Journ. Morph. 

 vol. i. p. 93. 



2 Cat. Cbelonians, Rhynchocephalians, and Emydosaurians in the British 

 Museum, p. 1. 



