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Show 162 PROF. G. B. HOWES AND W. RIDEWOOD ON [Mar. 6, with the absence of that definiteness of articulation between them, met with in nearly all other Anura. Pipa stands alone, as before stated (p. 161), with respect to the fusion of the postaxial carpal elements ; the resulting bone (pk) certainly includes the postaxial centrale and the ulnare, but we are unable to say whether our 5th carpale is or is not embodied. W e incline to the belief that it is not. Comparison of the young and adult feet (fig. 2, h) shows that this compound undergoes a shortening up during development. Its postaxial lobe (* of fig. 1) is of purely physiological significance and late in origin. Ventrally to the lunatum there is present a large lenticular bone (indicated by a dotted line in fig. 1). Baur has shown that, in the Urodele hind foot, an element of the tarsus 1 may be thus visible only on one surface, and hence we have studied with care the relationships of this element. It lies wholly out of the plane of the true carpal elements and is surrounded by tendinous tissue, while it is totally absent in the young specimen at our disposal. W e regard this combination of characters as proof that it is a sesamoid. The element marked po. i. is somewhat interesting. In the adult it is lenticular, and in articulation with both the naviculare (n) and the head of the 2nd metacarpal (2'). It might therefore appear, from its relationships, to be the carpal of the second digit. In the young specimen, however, it is exceedingly small (fig. 2) and wholly destitute of connection with the metacarpal; on comparison with Xenopus (fig. 4), in which the carpal of the 2nd digit is distinct, it is clear that it can only represent the carpal of the pollex 2, carpal 2 having, in all probability, become confluent with the naviculare. Examination of the young specimen (fig. 2) renders this doubtful, for the head of the 2nd metacarpal bears an enlargement, such as might conceivably have resulted from a confluence with its carpal. Upon this point we have been unable to obtain decisive evidence. Hind Foot. The most complete observations hitherto made upon the tarsus of this genus are those of Mayer (28). The hind foot is, like its fellow in front, very anomalous. The astragalus and calcaneus are widely separated and fused at their ends, leaving an elliptical hole in the middle ; the compound thus arising is only twice as long as broad. The outer edges of the two bones-i. e. the preaxial border of the astragalus and the postaxial one of the calcaneus-are ridged in such a manner that each bone is T-shaped in transverse section. The 4th and 5th metatarsals articulate directly upon the epiphysial end of the calcaneus, while the remaining three are as it were shut out from that of the astragalus by thin lamellae of bone. These lamellae are seen to be continuations of two out of three bones which lie on the ventral side of the tarso-metatarsal joint, and would appear 1 Centrale 2 of Necturus (1, p. 20). 2 The adult limbs at our disposal were skinned before they reached us, hence we are unable to make any definite statements concerning the pollex itself |
