MORPHOLOGICAL REVISION OF THE SUBORDER. 93 



Dimetrodon the anterior edge of the distal ends shows slight linear rugosities as if 

 there had been a loose squamous articulation with a small bone, an appearance which 

 suggests the possibility of a cleithruni (Nos. 4131 American Museum and looi 

 University of Chicago). The slender element described here seems after careful 

 study to be a distinct bone and not a splintered fragment ; if it is a distinct element 

 it is the cleithrum. The presence of a cleithrum in just such a rudimentary condition 

 is not surprising, but rather to be expected. 



The fore limb: There are several humeri in the lot, but all are of the same 

 type. It is highly probable that they belong with the same species as the skull and 

 pelvis. The articular ends are simple, without head or condyles, and were evidently 

 covered with cartilage. The proximal end is rounded in outline with no trace of an 

 articular face. The articular surface is widened next to the radial crest and is con- 

 tinued out upon it without interruption. The prominent radial crest leaves the main 

 part of the bone almost at right angles and the point of departure is marked on the 

 posterior side by a sharp ridge. The shaft is slender and subquadrate in section. 

 The entepicondyle is hardly larger than the ectepicoud)le ; the entepicondylar fora- 

 men is narrow and elongate. On the anterior face of the distal end is a ridge which 

 extends to and widens the distal surface in the position of the condyle for the radius. 



The radius and itlna are unknown unless a very thin fragment of a long bone 

 with an abruptly widened end represents the distal end of the radius. 



The radiale has almost the same form as in D. iiicisivus ; it has a broad prox- 

 imal surface for the end of the ulna and two faces meeting at an angle below for the 

 centrale i and 2. 



The itlnare is thin and disk-like, with two articular faces on the inner side for 

 the intermedium and centrale i. In the specimen it is slightly turned in position, 

 showing the facets. 



The intermedium projects well beyond the proximal end of the radiale and 

 ulnare. It had the same form as in Dimetrodon incisivus, but in the specimen it is 

 somewhat crushed. 



The centrale i and 2 are rather different from the same bones in Dimetrodon. 

 Centrale i is rather more cuboid, and centrale 2 seemingly did not send a process 

 inward to articulate with the carpale 3. 



Carpale 5 is absent, the others show nearly the same form and relation as in 



D. incisivus. 



The pelvis is nearly perfect ; the exact form of the acetabulum is hidden on both 

 sides by the heads of the femora which remain in the socket. The general fonn of 

 the pelvis is very similar to that of Dimetrodon incisivus, but the pubis and ischium 

 extended farther fore and aft, so that the symphysis is longer in proportion to the 

 height. The symphysis was quite deep and the two sides stood nearly vertical. 



The ilimn has a high crest with a strong posterior prolongation. The inner 

 face of the crest can not be seen, but there was evidently a strong attachment to the 

 sacral ribs. The upper edge of the acetabulum is rather prominent over a relatively 

 deep cup. 



The/M^w has the upper edge thickened into a strong ridge. The obturator for- 

 amen is just below the anterior edge of the acetabulum. The bottoni line is nearly 

 straight and the upper edge slants rapidly downward so that the anterior end is very 

 low and formed almost entirely by the thickened upper edge of the bone. The suture 

 between the pubis and ischium can not be traced. 



