OCR Text |
Show 168 PROF. G. B. HOWES AND W. RIDEWOOD ON [Mar. 6, form a capitato-hamatum as in the common Frog (3,4, k, fig. 20), upon which digits 3, 4, and 5 articulate. This family is unique in respect to the retention, subsequent to the incorporation of the 4th carpal in this bone, of that ligament (* fig. 20) which, in the simple forms, represents the 5th carpal (cf. p. 160). In Xenophrys (fig. 15) it will be seen that the fourth carpal is wedge-shaped postero-externally, extending over the inner half of the head of the fourth metacarpal; it is from this point that the ligament in question arises in both Xenophrys and Hyla, and we hold this as proof conclusive that the 4th carpal is, in the latter, represented in the manner described. Carpalia 1 and 2 are both distinct, the carpal of the pollex being elongated and slightly constricted in the middle (po. i. fig. 20). The pollex itself varies in degree of development ; its topographical value has been discussed by Cope and others (14, p. 200). Structurally it consists essentially of one piece (excluding its carpal) which may show traces of differentiation or constriction into two segments (po. ii., fig. 20). In the American Tree-Frogs with "external pollex " (Hyla albopunctata) the parts differ only in being very broad. Hind Foot (figs. 19, 21). As previously pointed out (p. 146), examination of the adult tarsus (fig. 19) would lead to the supposition that the tarsal of the hallux is absent, and that the two bones which are nearest related to the heads of the metatarsalia 2 and 3 represent their tarsals alone. In fact Gegenbaur (18, pi. 6. fig. 9) was led into this belief. Comparison of the larva shows that this is not the case, but that the outer of the two elements, when it first appears, has all the characters and relationships of the " cuboideum " 1 previously described in the Pelobatidce. This bone is present in all the Hylidce which we have examined, and it is characteristic of this and all the succeeding families. The astragalus and calcaneus are greatly elongated (cf. Hofmann, 21, pi. 14. fig. 10) in all, and their epiphysial cartilage is generally enlarged for special articulation of the 4th metatarsal (cf. fig. 19); as the result of this, these bones come to be inclined at a considerable angle to the metatarsals. The central portion of the epiphysis is ossified near the astragalar border (fig. 19) at the point of attachment of a powerful ligament, which is inserted at its outer end into the wall of the articular capsule and carries at its point of insertion a cartilaginous nodule. Iu old specimens this may become ossified, and we believe, for reasons previously given (p. 146), that it is an adaptive structure of no morphological importance, having nothing whatever to do with the missing 5th tarsal. There is no trace of it in the tadpole. The naviculare is present as a distinct element in all (n\ fio-s. 19, 19«), lying in a line with the pre-hallux tarsal (ph. i.). In Phyllomedusa it is unusually small, being barely larger than the tarsal of the hallux. 1 Cf. p. 146. |