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 Xeno- 

 phrys 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 topo- 

 graphical 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 "ex- 

 ternal 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. Com- 

 parison 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 " ' previously described in the 

 Pelobatidce. This bone is present in all the HylidcB which we have 

 examined, and it is characteristic of this and all the succeeding 

 families. 



The astragalus and calcaneus are greatly elongated (c/". Hofmann, 

 21, pi. 14. fig. 10) in all, and their epiphysial cartilage is generally 

 enlarged for special articulation of the 4th metatarsal (c/". 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 cap- 

 sule and carries at its point of insertion a cartilaginous nodule. In 

 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 («', figs. 19, 

 19 a), lying in a line with the pre-hallux tarsal (p/t. i.). In 

 Phyllomedusa it is unusually small, being barely larger than the 

 tarsal of the hallux. 



^ Cf. p. 146. 



