XIX.] MONOTREMATA. 327 



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 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 hallux is somewhat 

 shorter. The ungual phalanges are compressed, slightly 

 curved, and very sharp pointed. 



In the Echidna the astragalus is large, with an irregular, 

 broad, rounded, proximal articular surface, not divided by 

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

 ternal malleolus. The tuber calcmiei 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, 

 the others gradually diminish to the fifth. The ends of the 

 toes are turned outwards and backwards in the ordinary 

 position of the animal. 



The ungual phalanges of both extremities in the Mono- 

 tremata have a deep median groove, near the base of the 

 under surface, leading at its distal extremity into a foramen. 



