xix.] MONOTREMATA. 359 



In the Ornithorhynchus the proximal articular surface of 

 the astragalus is divided by a deep groove into two distinct 

 heads, one for the tibia, and the other for the fibula, the 

 latter being the larger of the two ; the inner side has a cup- 

 shaped socket, into which fits an incurved conical process 

 from the lower end of the tibia. The tuberosity of the cal- 

 caneum is broad and bifid at the extremity, and directed 

 not backwards, but towards the tibial side of the foot. In 

 the male there is an additional, large, flat, curved ossicle, on 

 the hinder and tibial side of the plantar aspect of the tarsus* 

 articulated chiefly to the tibia, which supports the peculiar 

 perforated horny spur characteristic of this sex, the function 

 of which has not yet been discovered. There is also a small, 

 rounded, supplementary ossicle, below the tibial edge of the 

 tarsus, near the articulation between the astragalus and 

 scaphoid. The metatarsals increase in length from the first 

 to the fifth. The phalanges are all rather long and slender- 

 The four outer toes are nearly equal ; the halluxis somewhat 

 shorter. The ungual phalanges are compressed, slightly 

 curved, and very sharp -pointed. 



In the Echidna the astragalus is larsje, with an irregular 



o O ' O 3 



broad, rounded, proximal articular surface, not divided by 

 a groove, and with a much less distinct fossa, for the in- 

 ternal malleolus. The tuber calcanei is directed forwards, 

 it is also bifid, and its external process is much longer than 

 the other and curved towards the plantar surface of the 

 foot. The spur of the male, and the ossicle which supports 

 it, are much smaller than in the Ornithorhynchus. The 

 metatarsals are shorter and broader ; they increase in length 

 from the first to the fifth. The hallux is very short, and has 

 a flattened, conical, ungual phalanx. The proximal and 

 middle phalanges are all very short and broad. The ungual 

 phalanx of the second digit is extremely long and falcate, 



