MORPHOLOGICAL REVISION OF THE SUBORDER. 



135 



condyle is flattened and is marked by a pit for the termination of the notochord ; its 

 section is triangular and the articular face is continued on to the lower surface. The 

 exoccipital and supraoccipital both take part in the formation of the condyle. The 

 exoccipital forms the sides of the foramen magnum and meets above it, shutting out 

 the supraoccipital. The lower surface of the basicranium is the same as in Dhuetro- 

 don incisivus. 



The axis: The best-preserved axis is No. 1060 University of Chicago (plate 25, 

 fig. 6). In the present specimen the spine is lost, but the centrum shows the usual 

 elongate form, the well-developed centantra and the wide face for the intercentrum on 

 the lower half of the anterior articular face. The transverse process rises from well 

 below the level of the anterior zygapophyses instead of on a level with them, as in 

 Dimetrodon incisivus. 



The third vertebra is very similar to 

 the axis, but with a narrow spine ; the 

 transverse process is longer and inclines 

 rather more to the rear, the face for the 

 rib looking backward as well as down- 

 ward. There is a deep pit at the base of 

 the spine on either side. 

 The Joiirth vertebra resembles the second, but the sides 

 of the centra are pinched in below the notochordal canal, so 

 that there is the beginning of the narrowing of the bottom 

 line to form a keel ; this is most prominent in the anterior half 

 of the centrum. 



The fifth vertebra is joined to the second, but is com- 

 pletely reversed in position in the specimen ; it has been placed 

 in the correct position in the figure. This vertebra shows the 

 typical characters of the posterior cervicals and anterior dorsals. 

 The centrum is shortened on the bottom line by the bending 

 back of the inferior edge of the anterior face, and the sides are 

 sharply pinched in just below the notochordal canal, forming 

 a high and thin keel. The anterior face of the centrum has 

 well-developed centantra and a wide face for the intercentrum. 

 The transverse process stands well out from the side of the 

 centrum. 



The sixth to the thirteenth are not connected, but have 

 been placed in position from their characters. The first three 

 are of similar form ; they are imperfect, but all show the sharp keel and shortened 

 bottom line. The transverse process stands well out, almost horizontally, from the 

 side of the centrum. 



The ninth has lost the lower half of the centrum, but preserves the neural arch 

 and the base of the spine. The anterior face of the centrum is rounded, having largely 

 lost the face for the intercentrum, but the centantra are still evident. The neural arch 

 is high and the faces of the zygapophyses are quite oblique, but not so much so as in 

 Dimetrodon dollavianus. The transverse process is short and stands out horizontally 

 from the side of the neural arch. The base of the spine presents very clearly some of 

 the characters of the species. It is smooth below, but a short distance up becomes 

 slightly rugose ; below the rugosity the spine is thin, but wide antero-posteriorly, and 



Fig. 60. — Left humerus ot D. 

 obtusiJens, No. 2152 

 Am. Mus. c/ii, ent- 

 epicondylar foramen. 



X 'A. 



61. — Posterior surface ot 

 the skull of D. obtusi- 

 Jens, No. 4062 Am. 

 Mus. X 73. 



Fig. 



