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Show 1891.] BRITISH REMAINS OF HOMCEOSAURUS. 171 incorrect to speak of a beak, and there is, I think, no evidence existence of uncinate processes in Hyperodapedon. Zittel's method differs from Lydekker's in this respect, that he entirely abstains from defining his suborders. W e 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 tbe 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 considerable difficulty to decide, because the two essentially distinctive characters separating the Plesiosauria from 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 Mesosaurus 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, tbe 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 Palceohatteria 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 Telerpeton, Sauroster-num, &c, are necessarily omitted. Order RHYNCHOCEPHALIA. Subord. 1. PROTEROSAURIA. Each transverse segment of the plastron composed of numerous pieces. Pubis and ischium plate-like. Fifth metatarsal not modified. Vertebra conically excavated at either end, with persistent notochord, all with intervertebral hypapophyses ; limb-bones without condyles ; humerus with entepicondylar foramen 1. Palceohatteriidce. |