Cope.] 4U [August 15, 



than long, and is separated by an angle only from the distal facet. The 

 latter is a little more than a half circle in outline, and joins one bone of 

 the second row, which I suppose to be the cuboid. The fact that it does 

 not articulate with the second element in that row, leads me to suspect 

 that the latter is the head of a fifth metatarsal. The external edge of the 

 bone thins out more rapidly at the distal than at the proximal extremity. 



The cuboid bone is pentagonal in outline, and square in transvere sec- 

 tion. It is not unlike that of the Amblypodous Mammalia. It has a 

 transverse proximal facet, and two distal ones which meet at an angle 

 about right. The fifth metatarsal is articulated with its posterior face; 

 and the fourth with the exterior distal face. The ectocunei'form articulates 

 with the interior distal face. The navicular bone is subtriangular in trans- 

 verse section, and with a subquadrate base articulating with the cuboid. 

 Its longitudinal and anteroposterior diameters are about equal. The distal 

 or metatarsal articulation of the entocunei'form is transverse and flat. 



The manner of articulation of the ankle-joint must have been different 

 from the usual reptilian type. The proximal extremities of the astragalus 

 and calcaneum combined are not too large to have received the distal ex- 

 tremity of the fibula, so that the tibial articulation must be sought else- 

 where. This may have been on the large distal facet of the anterior or 

 inner face of the bone. A part of this facet looks upwards and probably 

 supported the tibia, which was thus removed by a short space from that of 

 the fibula. The down-looking part of the facet, which is more distinct in 

 Embolophorus, must have articulated with a separate element. This may 

 have been a spur, such as exists in the known genera of the Monotremata ; 

 as the position is identical with that which bears this appendage in those 

 animals. It is quite evident that an element additional to those known in 

 the ordinary reptilian foot exists in the Clepsydropidae. 



The separation of the distal extremities of the tibia and fibula is not 

 usual among reptiles, but it is common in the salamanders, where the os 

 centrale comes between them. It is also evident that the subcyliudric 

 proximal part of the astragalus, which intervenes between the supposed 

 tibial and fibular articulations, represents that bone. 



The metatarsals are directed obliquely backwards as well as outwards, 

 as in Tachyglossus and Platypus. 



The following results may be derived from the preceding statements : 

 (1) The relations and number of the bones of the posterior foot are those 

 of the Mammalia much more than those of the Reptilia. (2) The rela- 

 tions of the astragalus and calcaneum to each other are as in the Mono- 

 treme Platypus anatinus. (3) The articulation of the fibula with both 

 calcaneum and astragalus is as in the Monotreme order of mammals. 

 (4) The separate articulation of the anterior part of the astragalus with 

 the tibia is as in the same order. (5) The presence of a facet for an articu- 

 lation of a spur is as in the same order. (6) The posterior-exterior direc- 

 tion of the digits is as in the known species of Monotremata. 



Thus the characters of the posterior foot of the Pelycosauria confirm the 



