REPTILIA : OPHIACODON 83 



As stated, there are numerous vertebrae in the collection that 

 have the general characters of the one figured in Plate XXXVIII, 

 Fig. 3. The larger part of the vertebrae, however, have not yet 

 been restored from their fragments, and it is quite possible that 

 among them later will be found two distinct types; one of them 

 belonging with Ophiacodon, the other with Sphenacodon. The 

 centrum of the posterior dorsal or lumbar vertebra shown in the 

 figure is rather shorter and stouter than others; it has a thin 

 keel below. ' The co-ossified rib has a slender, capitular process, 

 and a heavy tubercular one, uniting with both centrum and arch. 

 The rib, broken away in the specimen, was probably about fifty 

 millimeters in length. The spine is broad and flat, with thin 

 edge; its height is less than four times the diameter of the centrum, 

 though rather more elongated than in Varanosaurus. Anterior 

 dorsals in the collection have similar spines, as also caudals, and 

 it is because of this character that I assign the vertebrae to Ophia- 

 codon rather than to Sphenacodon, and which forces the conclusion 

 that the genus belongs among the Poliosauridae. 



PECTORAL GIRDLE AND EXTREMITY 



Clavicle. There are numerous clavicles in the collection, one 

 of the most complete of which I have figured in Plate XXXVI, 

 Fig. 4. It is a rather slender bone, only moderately expanded at 

 its mesial extremity. The outer or dermal surface of this part 

 is coarsely striate. This clavicle is of poliosaurid type, and quite 

 unlike the clepsydropid. 



Scapula. There are six or eight scapulae in the type collection, 

 all of nearly uniform size, but still unfortunately much broken 

 and fragmentary. The supraglenoid foramen pierces the supra- 

 glenoid fossa, not opening on the dorsal side of the scapula, as I 

 have observed it in all other theromorphous forms from the Ameri- 

 can Permian. There is no process on the upper margin of the 

 posterior coracoid, above the glenoid cavity, so conspicuous in 

 Dimetrodon. 



Humerus (Plate XXXV, Fig. 5). The humerus differs from 

 that of most species of Dimetrodon in its greater relative ter- 

 minal expansions, its shaft being less slender and shorter. The 



