1891.] BRITISH REMAINS OF HOMCEOSAURTJS. 171 



incorrect to speak of a beak, and there is, I think, no evidence of the 

 existence of uncinate processes in Hyperodapedon. 



Zittel's method differs from Lydekker's in this respect, that he 

 entirely abstains from defining his suborders. We are therefore left 

 to guess how the Proganosauria differ from the Rhynchocephalia 

 sensu stricto, and for what reason, for instance, the Champsosauridce 

 are placed in the former group rather than in the latter. Then, again, 

 as the Proterosauridce are included in the Proganosauria, why is the 

 latter term employed in preference to the name Proterosauria, -which 

 has priority? It is true this is but one instance out of many of 

 Zittel's disregard of the rules of nomenclature. But does this group 

 Proganosauria, originally founded upon Stereosternum, Cope, which is 

 now generally accepted to be synonymous with Mesosaurus, Gerv., 

 really belong to the Rhynchocephalia? This is a matter of con- 

 siderable difficulty to decide, because the two essentially distinctive 

 characters separating the Plesiosauria trom the Rhynchocephalia, in 

 the wide sense in which I would take these Orders, viz. the mode of 

 implantation of the teeth and the presence or absence of a lower 

 zygomatic arch in the skull, are not shown by the remains of Meso- 

 saurus at present known. But considering other characters, such as 

 the remarkable thickness of the ribs, the shape of the skull and teeth, 

 the absence of claws, I cannot but agree with Seeley and Lydekker 

 in regarding Mesosaurus as an early, generalized form of the Notho- 

 saurs, which gradually pass into the long-necked, marine Plesiosaurs. 

 The Proganosauria would nevertheless have to be considered as allies 

 of the early Rhynchocephalia, from which they diverged in the 

 Plesiosaurian direction, the Plesiosauria being, as I think all will now 

 admit, closely connected with the Rhynchocephalia. The point as 

 to whether Mesosaurus should be incorporated into the one or the 

 other of these two orders can only, I repeat, be decided on a precise 

 knowledge of the temporal arches and the dentition. 



As regards the grouping of the Rhynchocephalian families, I 

 hold that the Champsosauridce should not be placed in a suborder 

 apart from the true Rhynchocephalia, and less still together with 

 PalcBohatteria and Proterosaurus ; and that the latter genera well 

 deserve to rank as separate families. In fact, it seems to me that the 

 only satisfactory arrangement of the Rhynchocephalia, on the basis of 

 our present knowledge, may be expressed in the following synopsis. 

 Forms of which we know too little, such as Telerjpeton, Sauroster- 

 num, &c., are necessarily omitted. 



Order RHYNCHOCEPHALIA. 

 Subord. 1. Peoteeosauria. 



Each transverse segment of the plastron composed of numerous pieces. 

 Pubis and ischium plate-like. Fifth metatarsal not modified. 



Vertebrae conically excavated at either end, with 

 persistent notochord, all with intervertebral 

 hypapophyses ; limb-bones without condyles ; 

 humerus with en tepi condylar foramen 1. Pal<eohatteriidcB. 



