| OCR Text |
Show 158 PROF. G. B. HOWES AND W. RIDEWOOD ON [Mar. 6, his observation be reliable, this supposed centrale ought to be in younger specimens as a distinct element. W e have examined a large series of specimens, many of which were much younger than any which passed through Born's hands, and they showed without exception that the position of his " centrale" was occupied by the preaxial third of that element regarded by us as the postaxial centrale (k, fig. 9). W e can confidently assert that in no carpus under our hands does this show a trace of either segmentation into two, chondrification from two centres, or confluence of distinct elements l. In one specimen we have been able to detect a linear depression on the under surface of the cartilage, near the point at which Born's dismemberment appears to have occurred2 ; and we are strongly of opinion that this groove was present in his specimen, and that the section represented 3 by him passed through it. Indeed, his second assertion above cited seems, in itself, to confirm this belief. In the adults of certain forms the naviculare may lie, as already said, more or less completely in the proximal row, preaxially to the lunatum and in more or less definite articulation with the radius (cf. figs. 6, 18, 30). In others it may be as fully removed from the latter (figs. 1, 5, 7). It therefore becomes a question of vital importance as to which of these two conditions is the more primitive. In examining a large series of larvae of the Edible Frog, measuring each about 20 m m . from snout to vent, we were not a little surprised to find that the naviculare varied greatly in the extent to which it thus embraces, as it were, the lunatum. In the adult it comes to lie in the proximal row, nearly meeting the radius (n, woodcut fig. B, p. 174). In a young Bufo of about 20 m m . it was already in direct apposition with this bone. In Pipa (Plate VII. fig. 1), Xenopus (Plate VII. fig. 4), and most Hylidce (Plate VIII. fig. 20) it is strictly central; in one member of the last-named family however (Nototrema) it shows a tendency to become proximal in the adult. In the adult of Bombinator (Plate VII. fig. 7), and still more so in that of Discoglossus (fig. 6), it sends up a spur which approaches but does not nearly reach the radius. Appeal to development shows (figs. 5 & 9) that this spur is a late growth. The condition of the parts in the Pelobatido3 is deserving of special note. It will be seen that in those Anura in which the naviculare is most central in position the distal carpals are relatively small and reduced ; in Xenophrys (fig. 14) and Pelobates (fig. 18) these are, on the contrary, larger and more nearly uniform in size with the other elements of the carpus, so much so in the latter genus that the carpus has quite a Salamandriire aspect. Here too the naviculare is proximal 1 Ossification of this element invariably proceeds from one centre. 2 This becomes still more marked in certain forms in which our postaxial centrale is confluent with other elements (cf. p. 160). 3 There are other exceptional peculiarities about this carpus, for which we find no parallel in our own specimens. |
