107 



discovered forms, e. g. Sesamodon, are so mammal-like that it is really 

 doubtful if they ought not to be regarded as mammals. At present 

 for convenience a fossil form is regarded as a reptile if it retains the 

 angular bone no matter how small, and a quadrate bone no matter 

 how rudimentary. But Ornithorhynchus also seems to have a minute 

 angular, at least in the very young animal, and Pedetes still retains 

 an ossified quadrate. So near do the Cynodonts approach to mammals 

 that there can no longer be any reasonable doubt that the mammahan 

 ancestor was a small Cynodont reptile. When the osteology of the 

 Cynodonts is fully known no doubt complete answers will be found to 

 many of the puzzling problems of the mammahan skeleton. At present 

 it is necessary to look to the allied Anomodonts, almost every detail 

 of whose skeleton is now known. 



Fig. 1. 



Fig. 2. 



Manus and Pes of Oudenodon trigouiceps, BROOM. F fibula, R radius, T tibia, 

 U ulna, c centrale, i intermedium, F fibulare, ^j pisiforme, t tiluale. m ulnare. 1, 2, 3, 

 4, 5 carpalia and tarsalia. I, II, III, IV, V metacarpals and metatarsals. 



When we examine the manus and pes of the Anomodont genus, 

 Oudenodon, we see structures so very mammal-like that had they been 

 discovered apart from the skeleton they would doubtless have been 

 regarded as belonging to a mammal, as was the case with the Thero- 

 cephaUan genus Theriodesmus. The foot shows the origin of the 

 mammahan os calcis, astragalus, and navicular: how that the astra- 

 galus is the tibiale, alone and not the tibiale + intermedium as supposed 

 by Gegenbaur, that the os calcis is nothing but the fibulare and not 

 fibulare + fibulare sesamoideum as regarded by Allen Thomson, 

 and how that the navicular is merely a slightly displaced os centrale. 

 But what I wish especially to call attention to, is the peculiar de- 

 velopment of the 1st carpale and 1st tarsale. So elongated are they 

 that they might functionally be regarded as metacarpal and metatarsal. 



