PELONEUSTES. 35 



the cervical vertebrse 21-22 in number, the centra much shorter than wide. The 

 neural spines high and narrow. The limbs and limb-girdles generally similar to 

 those of Pliosaurus. 



This genus was established by Lydekker for the reception of two species of 

 Pliosaurian reptiles — Peloneustes ccqualis, Phillips, sp., from the Kimmeridge Clay, and 

 P. pltilarchus, Seeley, sp., from the Oxford Clay. Subsequently (Catal. Foss. Kept. 

 Brit. Mus. pt. iv. Supplement, p. 273) Pliosaurus evansi was also referred by him to 

 this genus. This species is larger than the other two, and in some respects seems 

 transitional between Peloneustes and Pliosaurus : a large mandible with an elongated 

 symphysis is here referred to it, and possibly some of the skeletons assigned to 

 Pliosaurus may also belong here. 



Middle Jurassic : England, Russia *. 



The following account of the structure of the skull and skeleton in this genus is 

 founded mainly on two fairly complete skulls and mandibles (R. 2679 and R. 3803) of 

 P. philarclms, and a nearly complete skeleton (R. 3318) of the same species, now 

 mounted in the Gallery of Fossil Reptiles. 



Skull (PL IV. ; text-figs. 11-13). — The skull in this genus has already been 

 described in some detail in the Annals and Magazine of Natural History, [C] 

 vol. xvi. (1895) p. 242, and the following account is founded partly on the description 

 there given and partly on the examination of several other well-preserved specimens, 

 particularly R. 2679 and R. 3803. 



The general outline of the skull is that of an isosceles triangle, the apex of Avhich 

 is blunt and the height about two and a half times the length of the base, that is 

 the width of the hinder end of the skull between the outer angles of the quadrates. 

 The basioccipital (b.oc; oc.c, text-fig. 11) is a very massiA r e bone; it bears the whole 

 of the occipital condyle (oc.c), with the exception of a very small portion of the 

 supero-lateral border borne by the exoccipitals. The condyle is oval in outline, the 

 long axis being transverse. The lateral regions of the bone are produced outwards and 

 downwards into a pair of very stout processes (l.p.), the truncated outer ends of which 

 look outwards and downwards and are much roughened ; they may have served in part 

 for muscle-attachments, but the anterior part at least was probably overlapped by the 

 pterygoids. The upper surface of the basioccipital bears two roughly quadrate 

 surfaces for union with the exoccipitals. These surfaces are concave and much 

 roughened ; they are separated in the middle line by a narrow ridge of bone, the 

 basioccipital having taken a very small share in the formation of the border of the 

 foramen magnum. 



The exoccipital (exo-opisth., text-figs. 11 & 12, D & E) in all specimens examined 



* A. EiabiniD, " Zwei Plesiosaurier aus den Jura imd Kreideablagerungen Russlands," Memoires du 

 Coinite Geologique, a. s. livr. 4/> (St. Petersburg, 1909). 



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