1905.] PRIMITIVE REPTILE PROCOLOPHON, 213 



equivalent to Pareiasauria. Cope (4) in 1889 placed it with 

 Palceohatteria, iMesosaurus, 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 Procolopkon in the Family 

 Pariotichidaj in the Order Theriodontia. In the_ same year 

 Lydekker (6) agreed with Seeley in placing it in a distinct Sub- 

 order 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 Pcdceohatteria, and Osborn (9) has adopted 

 a somewhat similar view, placing the Order Procolophonia in the 

 Superorder Diaptosauria. Bonlenger (10), the mostrecent 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 Gotylosauria, an order which he believes 

 to be descended from the Pareiasauria. 



In the past, much of the difierence of opinion arose from an 

 imperfect knowledge of the structure of Procolojjhon ; at present it 

 arises mainly from the imperfection of our knowledge of the struc- 

 ture 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 palseontology. 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 Gotylosauria ; 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. Sphenodon, 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 



