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Show 1888.] THE CARPUS AND TARSUS OF THE ANURA. 171 the radius (fig. 25). The pollex consists of two short segments, borne upon an equally insignificant carpal- (po. i.-iii.), the whole standing out at a right angle to the long axis of the fore foot. In the male the head of the second metacarpal is, as Mayer has shown (27), immensely enlarged for support of the horny clasper (** fig. 25). It would appear, at first sight, that the displacement of the naviculare might have been the direct outcome of this enlargement; but if so, the reduction of the 2nd carpal to the condition of an absolutely insignificant nodule (2) becomes unintelligible, for it certainly would appear that that must have resulted from the same cause. We find, in the young specimen at our disposal, that this is really the case, for in the absence of the clasper the 2nd metacarpal is relatively larger. It is interesting to note further, that while, in this specimen, the naviculare does not quite reach the radius, the pollex is proportionately much stouter than in the adult. On consideration of these facts we incline to the belief that the displacement of the naviculare was originally associated with the enlargement of the pollex, in a manner similar to what is seen in Pelodytes (fig. 11), and that with the reduction of the pollex its connection with the radius became lost, only to be resumed again on the great enlargement of the 2nd metacarpal. In the male of Limnodynastes the preaxial border of the second metacarpal bears an irregular bony crest, like that of the Common Frog. We find, however, that the pollex never here fuses with this, as it may do in the last-named genus. Hind Foot (figs. 24, 27, and 29). The typical condition of the Cystignathid hind foot is exemplified in Cystignathus and Limnodynastes (fig. 29), and, as the figure shows, it is, in general, Frog-like-i. e. tarsalia 2 and 3 are fused to form a single cuboideum (2 3 ) . In Leptodactylus this splint is shortened up, so that the hallux-tarsal nearly equals it in size. In Ceratophrys the hallux-tarsal is absent, having either fused with the naviculare or disappeared, as believed by Born (3, p. 441) for Rana. The calcar is, like the pollex, subject to no inconsiderable variation, as might indeed be expected of a family whose members lead such diverse modes of life. In Ceratophrys (fig. 24), Cystignathus, and Limnodgnastes (fig. 29), we have only been able to recognize two segments, the basal one of which represents the tarsal (ph. L), while in Leptodactylus the full complement of four pieces is reached. Pseudis (fig. 27) bridges over the interval between these two conditions, in that a small first phalanx is present (p.h. iii.). In this animal the middle or metatarsal segment greatly exceeds the basal one in length, and its proximal outer border is enlarged; in Limnodynastes this enlargement becomes more obvious, leading up to the condition seen in Ceratophrys ornata (fig. 24), where it assumes the form of a retral spurl. 1 This is very feeble in a smaller specimen of C. americana examined. |
