154 Bulletin American Museum of Natural History. [Vol. XXVII, 



II and 11 overlapping I. (Primitive.) The nianus bears no marked sug- 

 gestion of former opposibility in digit I. In Echidna the proximal ends of 

 the metacarpals are more in line. The manus of Echidna as a whole rather 

 resembles that of Oudenodoti (cf. Broom, 1901, pi. xvi). 



Hind limb. — Pelvis. The most noteworthy feature (aside from the 

 epipubic bones) is the small round thyroid or "obturator" fenestra; the 

 ventral aspect of the pelvis is rather "plate-like" /. e., as in Permian Reptiles 

 (p. 115). (Primitive.) The pectineal tubercle as in Marsupials and certain 

 Placentals is well developed. The ilium in Echidna is fully mammalian in 

 character, i. e., it is a trihedral bar, rather short, and with a reduced postero- 

 dorsal projection (contrast Therapsid reptiles, p. 118, 119). In Ornithorhyn- 

 chus the dorsal border of the ilium is everted. In Echidna the acetabulum 

 is perforate, as in Sauropsida (not so in Ornithorhynclms). The acetabular 

 notch, for the ligamentum teres is lacking. 



Femur. In Ornithorhynchus the inclination of the neck and head of 

 the femur to the shaft is much less than in ordinary mammals. The greater 

 and lesser trochanters are on directly opposite sides of the shaft. In Echidna 

 the first and third trochanters are continuous and are represented by a 

 single plate-like extension. Comparison with other archaic mammals 

 indicates that this may be a primitive character. The patellar surface in 

 both genera is flat and not grooved as it is in typical mammals (Primitive). 

 The femur in Ornifhorhynchus is turned outward much more than in other 

 terrestrial mammals. (Partly primitive.) 



Fibula. The fibula is remarkable for the dorsal extension above the 

 level of the distal end of the femur. In Marsupials and certain Insectivores 

 (Microgale, p. 251) the fibula also has a proximal expansion, which however 

 never attains the development that it does in Monotremes. 



Pes. The pes of Monotremes comprises the same elements as that of 

 other mammals but the tarsal bones are modified in a very extraordinary 

 manner. The tarsal spur (vestigial or reduced in females) is borne on the 

 postero-inferior border of the large quadrangular tibial sesamoid, which in 

 tui'n articulates with the internal end of the tibia and with the internal inferior 

 side of the tarsus. The astragalus is flattened inferiorly; its vipper surface 

 bears a high postero-external convexity for the support of the fibula, and a 

 lower, antero-internal concavity for the tibia. The calcaneum is a squarish 

 depressed bone, and points backward and downward, parallel to the long 

 axis of the body and at right angles to the pes, which is turned outward. 

 In Echidna the tuber calcis forms a downwardly pointed hook and the 

 whole calcaneum is much smaller than the astragalus. The latter has a 

 very large convex postero-external condyle bearing the fibula and most of 



i 



