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Show 1905.] PRIMITIVE REPTILE PROCOLOPHON. 213 equivalent to Pareiasauria. Cope (4) in 1889 placed it with Palceohcitteria, Mesosaurus, and a number of other primitive types in the Order Proganosauria, one of the subdivisions of his group, the Theromora. Zittel(5) in 1890 put Procolophon in the Family Pariotichidse in the Order Theriodontia. In the same year Lydekker (6) agreed with Seeley in placing it in a distinct Suborder Procolophonia of the Order Anomodontia. In 1892 Seeley (7) removed it from subordinal rank, and regarded it as the type of a distinct family of the Pareiasauria, and this latter view of Seeley's has received the support of most recent writers. Two years ago I (8) expressed the opinion that Procolophon should be placed somewhere among the primitive Rhynchocephalians- possibly not far from Palceohatteria, and Osborn (9) has adopted a somewhat similar view, placing the Order Procolophonia in the Superorder Diaptosauria. Boulenger (10), the most recent writer on the subject, however, removes Procolophon entirely from all close relationship with the Rhynchocephalians and makes it the type of a family of the Cotylosauria, an order which he believes to be descended from the Pareiasauria. In the past, much of the difference of opinion arose from an imperfect knowledge of the structure of Procolophon; at present it arises mainly from the imperfection of our knowledge of the structure of the other early reptiles to which it shows resemblances. Within the last few years, the view that the Reptiles early divided into two distinct phyla has been steadily gaining ground and at present it has the support of the majority of workers on vertebrate palaeontology. In the one branch are the mammal-like reptiles; in the other the lizard-like forms. Osborn and most of the American authorities consider that both phyla have sprung from a primitive reptilian order, the Cotylosauria; but Boulenger believes that the common ancestor was a Stegocephalian Batrachian. If we compare an early type of the mammal-like group, e. g. a Therocephalian, with a generalised type of the lizard-like phylum, e. g. Sphenoclon, we find, that though there are many differences, there are many striking resemblances, and that they have a large number of characters in common, which are not found in any Batrachian. The structure of the palate is almost identical in the two types, though quite unlike that of the Batrachian. A well-developed columella cranii is found in both Anomodonts and Rhynchocephalians and both have an occipital condyle largely formed by the basioccipital bone. The axis, atlas, and pro-atlas are essentially similar in both phyla so far as known, and both have a costo-sternum. Unfortunately we cannot study the soft parts or the ontogeny of the Anomodonts, but in the closely allied Mammals we find so very large a number of characters which are common to the typical Reptiles and are unknown among the Batrachians, that the advisability of uniting the Mammals with the Birds and Reptiles to form the Amniota is generally admitted. To account for the characters which the Mammals and Reptiles have in common, we must either assume that the characters were |