PERMO-CARBONIFEROUS VERTEBRATES FROM NEW MEXICO. 



29 



17. — A, Diasparactus zenos Case, left tibiale from the 

 front, X }^; B, left tibiale of same from the rear, X H; 

 C, carpus of an unidentified species of Diadectes, in the 

 collection of the University of Chicago: /, tibia; /, fibula; 

 a, astragalus (tibiale); c, calcaneum (fibulare). 



and slightly forward; the posterior, smaller face looks directly outward. If the 

 bone is viewed from the inner side its outline is quite suggestive of the tibiale of 

 the Cotylosauria, but it seems that this is more of a coincidence than otherwi.se 

 and that the true position of the bone is different from that in the order mentioned 

 and that the faces articulate in a different manner. Allowing the bone to remain 

 in the horizontal position in which it was found, which seems quite correct, it 

 lies nearly across the tarsus, its inner end taking a small part in the articulation 

 with the fibula and its large, outer, oblique face accommodating the tibia. 



The fibulare is oval, with a broad face on the upper side for the fibula and 

 a narrower one on the inner side 

 for the tibiale. The lower and 

 outer edges are thinner, but still 

 quite broad. The distal row con- 

 sists of four bones. Tarsalia I, 

 II, III are small, oval, flattened 

 nodules with no articular surfaces 

 and indicate the presence of a con- 

 siderable mass of cartilage in the 

 foot. Tarsale IV is larger, roughly 

 triangular, with broad edges and a 

 deep concavity on the lower sur- 

 face. There are no traces of cen- 

 trale bones. The attitude of the 

 tarsus in this and related fonns was far different from that in the Pelycosauria or 

 in the modem Lacertilia; the foot was not set obliquely to the axis of the foreleg, 

 but was continued directly forward. 



The phalanges are similar to those described from the front foot. The meta- 

 tarsal of the first digit is short and very wide, that of the fourth digit is longer than 

 any in the front foot, and the whole digit was also, apparently, longer. The first 

 phalange of this digit is short, but much wider and heavier than the metatarsal. 



The vertebral column: The condition of the vertebral column has been de- 

 scribed in the general account of the specimen. The atlas is wanting; in only one 

 specimen of the Diadectidae (No. 1075 Univ. Chicago), the nearest related group, 

 has this bone been observed. It is a fiat disk, perforated by the notochordal canal, 

 with detached neural arches. The arches were weak, with small and weak posterior 

 zygapophyses for attachment with the axis. It is not known whether they were 

 divided, as in the Stegocephalia (young stages of Eryops and Trimerorhachis) , but 

 in all probability they were. The atlantal intercentrum is preserved in the speci- 

 men of Diasparactus; it is thick and heavy, with the anterior articular face for the 

 occipital condyle wider than that for the axis. 



The axis: This is not in connection with the rest of the vertebral column or 

 the skull, but is identified by the neural spine; this is thin and elongated antero- 

 posteriorly, with the posterior edge nearly straight and the anterior edge slanting 

 downward and forward to project over the atlas, quite as in Dimetrodon and Dia- 

 dectes. The sides of the neural arches are not expanded and convex as in the 

 dorsal vertebrae. There is a short transverse process for the attachment of a well- 

 formed rib. The centrum is small, with nearly circular faces; the sides are smooth, 

 somewhat contracted, and meeting in a sharp median keel below. The lower edge 

 of the anterior face is beveled by an articular face for the axial intercentrum, which 

 is nearly as large as the first intercentrum. 



