1881.] »^" [Cope. 



looks also from its free inferior side lilce an ungual phalange, but is flatter 

 than is characteristic of this family. There are three metatarsals distal to 

 the navicular, which are well accommodated with articular facets on the 

 distal extremities of the three bones in question, so that their identifica- 

 tion as the three cuneiforms, is probably necessary. The two remaining 

 metatarsals are articulated, the fourth to the exterodistal facet of the 

 cuboid ; and the fifth to the exterior side of the cuboid. The third, 

 fourth and fifth metatarsals are directed at an obtuse angle posteriorly 

 from the long axis of the astragalus. 



This structure is more mammalian than any form of foot yet known 

 among reptiles, and agrees with the indications of mammalian character 

 described as existing in the long bones of the limbs by Owen and by 

 myself. 



The astragalus is an oblong bone with one long straight side, viz., that 

 which is in contact with the calcaneum. This side has two facets for 

 articulation with the calcaneum, which are separated by a groove, which 

 forms a foramen when the two bones are in place. The proximal ex- 

 tremity of the bone is much smaller than the distal, and is subround. 

 The proximal half of the bone would be nearly cylindric were it not for 

 the truncation caused by the calcaneal facet. The distal half of the bone 

 is robust, and is surrounded on all sides by facets. These are the external 

 or calcaneal, the distal or navicular, and the internal which is larger than 

 the other two together. The first two are oblong and truncate, the navic- 

 ular twice as large as the calcaneal, its transvere much exceeding its 

 anteroposterior diameter. Tiie internal facet already mentioned, covers 

 the internal face of the distal half of the astragalus, which projects fur- 

 ther inwards than the proximal half, rising abruptly from it. The facet is 

 continuous with the navicular, and is at right angles to its plane. It 

 widens proximally, and its proximal border is deeply notched. Its surface 

 is convex from back to front, but not strongly so. In the astragalus of a 

 species of Dimetrodon, it is divided by an angle into two facets, the two 

 faces thus produced being nearly at right angles to each other. This in- 

 ferior part of the facet is continued into a prominent border which is more 

 or less roughened. A rounded tuberosity of the inferior face of the bone 

 occupies the space between this border and the calcaneal border, so 

 approaching the notch already described, as to cause a groove to proceed 

 from it posteriorly and inwards. I described the corresponding bone in 

 the Clepsydrops collettii (Proceeds. Phila. Academy, 1875, p. 409) as a 

 possible coracoid. 



The calcaneum has its postero-external edge broken in the specimen of 

 Clepsydrops natalis described, but is probably a semidiscoid bone, with its 

 straight margin applied to the astragalus. This margin presents a median 

 flat elongate-oval facet, which is separated by grooves from a facet at each 

 end. The proximal facet is the narrower, and passes by a curve into the 

 proximal extremital facet, which is adjacent to the corresponding proximal 

 facet of the astragalus. The distal internal facet is triangular and wider 



