134 REVISION OF THE PELYCOSAURIA. 



Dlmetrodon platycentrus sp. Case (p. 54). 



Characteristic specimen : No. 4065 Ain. Mus. The type. 



This species is very similar to Dimetrodon niacrospoudylits^ but the bottom line 

 of the centrum of the posterior dorsals and anterior lumbars, instead of being drawn 

 to a keel, is broad and rounded and even flattened. This is more apparent on the 

 posterior portion of the column, so that on the second lumbar vertebra the bottom of 

 the centrum is nearly flat transversely and is marked by rugosities which have a 

 distinct tendency to a radial arrangement. The bottom line is concave antero- 

 posteriorly, and the edges of the centra are veiy prominent. The articular surfaces 

 are reflected on the sides of the centrum and are beveled on the lower edge, leaving a 

 large space for the intercentrum. 



Measiirerncnls. 



mm. mm. 



Length, bottom line of — Length, bottom line of — 



2d lumbar 21 Penultimate dorsal 25 



ist lumbar 23 Antepenultimate dorsal 26 



Last dorsal 23 



Dimetrodon obtusidens Cope (p. 54, plate 25). 



Theropleura obtusidens Cope. 



Characteristic specimens : No. 4007 Am. Mus.: The type. No. 4062 Am. Mus. 

 Nat. Hist. Cope, coll. : Homeotype. The basicranium and a series of twenty-one or 

 twenty-two vertebrae, not all in series; numerous fragments of spines. From the 

 mouth of Beaver Creek, a tributary of the Big Wichita river, Texas. No. 1060 

 University of Chicago : A single axis; from Texas. No. 2152 Am. Mus. Nat. Hist, 

 Cope, coll. : A humerus ; from Texas. 



The type sjDecimen consists of a number of mixed bones. The jaws originally 

 described by Cope are the two lower jaws, and on the other side of the mass is seen 

 the posterior face of the skull, with the elevated postorbital region of a typical Dime- 

 trodon. The "palatal dentigerous bone" described by Cope is part of the pterj'goid, 

 but it is not at all certain that it belongs with the skull, as it seems too small. The 

 "rhombic" form described by Cope is due to the fact that the pterygoid approaches 

 in form that of the Pol iosaur idee ; the external process of the pterygoid does not stand 

 upright at right angles to the main portion, but lies at a low angle to it, so that the 

 external process of the pterygoid and the palatal portion fonn the rhombic area. The 

 external face of the external process is very narrow, and on the lower edge there are 

 but three teeth in sockets ; the patch of small teeth begins at the inner edge of the 

 process, instead of after an interval as in Dimetrodon incisiviis. 



With the specimen are three humeri, one of which belongs to some member of 

 the DiadectidcE ; the other two probably belong with the skull, as they are of the same 

 size. They are of the same general form as Dimetrodon incisivus, but the radial pro- 

 cess begins much farther down on the side of the bone and is separated from the head 

 by a considerable interval. A scapula is also preserved in the mixture of bones, which 

 has the same form as Dimetrodon incisivus. 



The vertebral column is represented by several vertebrae probably belonging with 

 the skull. By direct comparison the vertebral column. No. 1838 Am. Mus., is seen to 

 belong to the same species and is described as Dittietrodon obtusidens. 



No. 4062: The base of the skull is nearly complete, lacking only the distal ends 

 of the paroccipitals and a fragment just over the foramen magnum. The occipital 



