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Show 216 DR. R. BROOM ON THE [Mar. 21, and is only a roofing scale of bone. It is probably the homologue of the bone usually called " epiotic " in Stegocephalians, but it is evidently a membrane-bone and not developed from the auditory capsule, and hence not a true epiotic. A similar bone is found in Pariotichus, but is lost in all the higher forms. If we omit from consideration this so-called epiotic, we find the parietal, postorbital, squamosal, jugal, quadrato-jugal, and quadrate bones having exactly similar relations to each other in Procolophon and Sphenodon, the chief difference being that there are two fenestra? in the latter form. The condition of the teeth I do not look upon as a character of much importance in the matter of broad classification, but the teeth in Procolophon are by no means thecodont in the ordinary sense. Owen, in 1876, rightly stated that " the base of the tooth seems to be confluent with the osseous substance of the jaw " ; and Lydekker, in 1890, stated that the marginal teeth are " completely anchylosed to the bone." Most likely in the young condition the teeth developed in sockets, but in the adult they must be regarded as much more acroclont than thecodont. Boulenger states that " the thecodont dentition, the absence or great reduction of the plastral bones, and especially the presence of ossified precoracoids, are characters which are opposed to the association of the Procolophonia with the Rhynchocephalia." But, as has just been mentioned, the teeth cannot be regarded as thecodont, being nearly as typically acroclont as in Sphenodon; the plastral bones are quite as well developed in Procolophon as in most Rhynchocephalians ; and the presence of ossified precoracoids in Procolophon cannot be urged as a reason for removing it from association with the ancestral Rhynchocephalians, since the early Rhynchocephalians must have had ossified precoracoids, if the Pely-cosaurians are descended from them, as is believed by Boulenger. If Procolophon is to be removed from a position near the ancestral Rhynchocephalians and placed near Pareiasaurus, it must be for other reasons than those advanced by Boulenger. Some additional evidence in favour of placing Procolophon in the Diaptosauria, or at least among the ancestral Diapsidan reptiles, is to be found in the striking resemblance which it bears in many points of structure to Mesosaurus. Unfortunately, the skull of Mesosaurus is imperfectly known, but all the parts of the skeleton that can be compared are fairly similar to those in Procolophon. There is an ossified precoracoid, anchylosed, however, to the coracoid and scapula, and the pubis and ischium closely resemble those of Procolophon. Though the carpus is imperfectly ossified, there can be little doubt, when that of Stereosternum is considered, that it has been modified from a Procolophon-like type. The tarsus is almost identical in structure with that of Procolophon-the intermedium uniting similarly with the tibiale, and a foramen being formed between the conjoined bone and the fibulare. The plastron is closely similar in the two forms. I have recently tried to show (Trans. S. Afr. Phil. Soc. |