Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/156

96 The lower canines (ib. fig. 5, c′, c′) had risen, as in Machairodus, immediately in front of the upper ones, and presented the same inferiority of size; hut they are divided, in Cynodraco, by a toothless interval, or "diastema," from the lower incisors (ib. i, 1, 2, 3, 4). In this character, as in the number of incisors, the South- African Karoo fossil resembles the Marsupial genus Didelphis. The lower incisors are subequal, subcompressed, and elliptical in transverse section, at least at the base of the crown, with the long-axis of the section directed from the fore to the hind part of the alveolar border; and they are close-set, as in carnivorous mammals. As in these, also, the dentine, in both canines and incisors, is of the hard unvascular kind, and the enamel as distinct in tissue and as thick.

The decrease in size is from the 1st to the 4th; but the degree shown in the fossil and fig. 5 may be due to section at different heights from the base.

Associated with this fossil, or from near the same locality, was a larger oblong block of the same matrix, with the ends of a long bone partially visible. Out of this block an entire humerus was developed (Pl. XI. figs. 6–9, half nat. size). It is of a left fore limb, in length 10 inches 6 lines, with some loss by abrasion of both articular extremities, the shaft showing well-marked developments for muscular attachments and other characters unusual or unknown in the Reptilian class.

The breadth of the distal end—the extension of strong ridges from both the outer (e, e′) and inner (f,f′) sides, just above the elbow-joint, indicative of strong supinators, flexors, and extensors of the forearm and paw—the modification of the articular surfaces of that end, better preserved than those above, for the combination of due attachment of two bones of the forearm with freedom of motion, not only in the bending and extending, but in rotating on each other, so that the paw could be turned "prone" and "supine," whereby its application as an instrument for seizing and lacerating is advantaged,—add to this the structure, hitherto known only in the Mammalian class and preeminently in the feline family, of a defence of the main artery and nerve of the forearm from compression during the action of the above-named muscles by a strong bridge of bone (h) spanning across them, furthermore the extensive and powerful ridge (b, b′) at the proximal half of the humerus for the attachment of arm-muscles, especially the deltoid,—the combination of these characteristics, which Cuvier dwells upon in contrasting the humerus of the feline and bovine mammals, are here exemplified in a fossil homologue, from a formation of the Triassic or Permian division of geological time.

Extending, however, the comparison of the present humerus beyond the salient features above defined, the head of the bone (fig. 8) differs from that of the feline humerus in being broader transversely, instead of from before backward; the articular part is oblong and narrow, not hemispheroid nor nearly so convex; there is no elevation of an outer transverse tuberosity. The representative of the