160 Dr. C. W. Andrews — Skull and Jaw of Peloneustes. 



former of these genera in having one of the anterior thoracic legs 

 enlarged and strongly spinose ; it is said to differ in having no 

 thoracic exopodites, but even if this should prove to be the case it 

 would not outweigh the important points of resemblance. 



A. provisional diagnosis of the genus Pleurocaris may be given as 

 follows: — Eumalocostraca without a cai'apace, with at least seven of 

 the thoracic somites distinct and provided with horizontally expanded 

 pleural plates ; with at least seven pairs of the thoracic appendages 

 not greatly differing in size ; with the abdomen longer than the 

 thorax; with the telson and nropods forming a tail-fan, the former 

 distinct from the last somite and tapering to a sharp point, the latter 

 with long narrow rami. Probably belonging to the division Syncarida 

 and closely allied to the genus Acanthotelwn, Meek & Worthen. 



It is perhaps deserving of mention that the transversely ridged body- 

 somites of Pleurocaris give it a superficial resemblance to certain 

 Myriopods (Diplopoda) occurring in the same strata, and one 

 occasionally finds fragments, destitute of appendages', which can witli 

 difficulty be referred to one or the other. 



IV. — On the Structure of the Hoof of the Skull and of the 

 Mandible of Peloneustes, with some remarks on the Plesio- 

 satjrian Mandible generally. 



By C. W. Andrews, D.Sc, F.E.S. (British Museum, Natural History). 1 



IJT the course of the preparation of the second part of the Catalogue 

 of the Marine Reptiles of the Oxford Clay in the Collection at the 

 British Museum, it has been necessary to examine in detail the 

 structure of the skull and mandible of the Pliosaurs, especially of 

 Peloneustes philarchus, several excellent specimens of which are 

 included in the Leeds Collection. In the course of this examination 

 several peculiarities have been observed, which on the one hand tend 

 to reconcile the conflicting views as to the structure of the skull roof 

 that have been put forward, and on the other serve to correct the 

 interpretation of the elements of the mandible given in the first part 

 of the Catalogue. 



In the Plesiosaurian skull one of the most difficult points to 

 determine is the precise position of the suture between the parietals 

 and f rontals, and the relationship of those bones to the pineal foramen. 

 In Murcenosaurus, Cryplocleidus, and Tricleidus this suture runs across 

 the front of the pineal' foramen, the parietal overlapping the frontal in 

 such a way that while the former may completely enclose the foramen 

 on the outer surface of the skull (as in 3Iur<enosaurus and Cryptocleidus), 

 the frontals extend back heneath the parietals, and form the anterior 

 border of the opening on the cranial surface. In the type-specimen of 

 Tricleidus secleyi, in which the various elements are disarticulated 

 from one another, it can be seen that the anterior part of the border 

 of the pineal foramen is formed by the frontals alone. In Peloneustes 

 the arrangement of these bones is very different. In this case the 

 forward extension of the parietals over the frontals anterior to the 



1 Published by permission of the Trustees of the British Museum. 



