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



12. On the Structure of the Plesiosaurian Skull. By Charles W. 

 Andrews, Esq., B.Sc, F.G.S. (Read February 26th, 1896.) 



[Plate IX.] 



The structure of the skull of the Liassic Plesiosauria has been 

 discussed by many writers, but the various accounts that have been 

 given of it are incomplete, and often differ one from the other in 

 important particulars, doubtless owing to the fact that in most 

 cases the specimens examined are much crushed, and are embedded 

 in the matrix, so that only one aspect is visible. In the National 

 Collection there is, however, a fine skull of Plesiosaurus macro- 

 cephalus, which has lately been almost completely cleared from the 

 matrix, so that it exhibits both the upper and under surfaces ; this 

 specimen, though it has been subjected to a slight vertical com- 

 pression which has caused some fractures and dislocations, gives a 

 fairly clear idea of the general arrangement of the constituent bones, 

 and, since it throws light on some obscure points, seemed worthy 

 of the following brief notice. Certain other specimens, which are 

 of assistance in some difficulties, will also be referred to. 



In 1838 Owen x figured and described the upper and lateral regions 

 of the skull of P. macrocephalus, and in 1881 Sollas 2 described under 

 the name P. brachycephalus some portions of the head of a specimen 

 probably referable to the same species. Neither of these writers 

 bad an opportunity of examining the palate, and it is this region, 

 therefore, that is more particularly considered here ; while, in the 

 structure of the rest of the skull, only such points are noticed as 

 seem to add to, or to be at variance with, the descriptions already 

 published. 



The specimen (PI. IX.) under consideration is from the Lias of 

 Lyme Regis, and was referred to Plesiosaurus macrocephalus by 

 Mr. Lydekker. 3 The occipital surface is still somewhat obscured by 

 adherent matrix, and has the anterior cervical vertebras attached to 

 it, although the atlas has been dislocated from its articulation with 

 the occipital condyle. 



The bones of the palate (PI. IX. fig. 1), though somewhat dis- 

 placed from their natural positions, are, with the exception of the 

 transverse bone, fairly well preserved and distinct, so that their 

 form and relations can easily be made out. 



The basioccipital (b.oc.) bears the whole of the nearly hemi- 

 spherical occipital condyle, and carries on either side a stout, 

 outwardly- directed tuberosity, the truncated end of which looks 

 outward. In the Plesiosauria the whole of these tuberosities 

 is formed by the basioccipital, but in most reptiles the basisphenoid 

 enters into their composition. 



1 Trans. Geol. Soe. ser. 2, vol. v. pt. iii. (1840) pi. xlv. 



2 Quart. Journ. Geol. Soc. vol. xxxvii. (1881) pi. xxiv. fig. 1. 



3 Cat. Foss. Rept. Brit. Mus. pt. ii. (1889) p. 268, no. 49202. 



