1910.] Astragalus in primitive Mammals. ' 455 



(1) Its oblique position on the side of the calcaneum. 



(2) Its very broad trochlea, with poorly defined condylar ridges. 



(3) Its short neck, not produced interiorly, especially at the infero- 

 internal angle. 



(4) The relatively external position of the sustentacular facet and its 

 continuity with the navicular facet. 



(5) The absence of a cuboid facet. 



Placentals. There is some doubt as to which is the most primitive type 

 of astragalus known among Placentals but the type with very broad trochlea, 

 short neck and convex head ^ seen in the most ancient Rodents, Edentates, 

 Basal Eocene Periptychids and Homalodofheriam appears to be morpho- 

 logically nearer the Marsupial type than does the relatively long narrow astra- 

 galus, with narrow trochlea and slender neck seen in the smaller Creodonts, 

 Condylarths {Eiiprofogonia) and certain Insectivores (e. g., Microgale, 

 [Fig. 19, C^, p. 250] Sorex, Twpaia). 



Rodents and Edentates. In these orders, as well as in some of the Notoun- 

 gulata (e. g., Homalodotherium) the astragalus usually has a very "\\'ide 

 low-keeled trochlea a short oblique neck and a very convex or even globular 

 head, and the ectal and sustentacular facets are more or less parallel, oblique, 

 and sei)arated by a deep groove which sometimes (e. g., Orijcteropus Homa- 

 lodotherium) ends in an astragalar foramen. 



The cuboid contact is variable but usually absent. The astragalus of 

 the Eocene Rodent Paramijs (p. 327) approaches the Insectivore type. 



Insectivores. In the more primitive Lipotyphla (])p. 252, 254) the astra- 

 galus, as stated above, is primitively long and slender, at least in certain 

 Zalambdodonta (Fig. 19, p. 250), Erinaceidae and Soricidoe; but has a wide 

 trochlea and short neck in Pantoles-tes (Matthew). The trochlear crests are 

 developed to a varying extent but the inner is often subequal with the outer 

 one. A vestige of the astragalar foramen appears to be present in Erinaceus 

 (p. 252). 



Creodonta. In this group the trochlear ridges are variable; in Patrio- 

 felis the ridges are but faintly expressed, in Hijo^nodon and the Mesonychidse 

 they become very pronounced. The astragalar foramen is usually present 

 but is lost in the Mesonychidje; it appears to have resulted from the bridging 

 over of a notch in the postero-superior rim of the trochlea, which notch was 

 probably continuous with the groove between the ectal and the sustentacular 

 facets (see above, p. 454). An astragalo-cuboid contact appears to be a 

 primitive character for the Creodonta (as it is also for the Taligrada), since 



1 The extension of the antero-dorso-internal part above the trochlear facet appears to be 

 another primitive character seen especially in Periptychids, early Creodonts and more ancient 

 Patagonian Ungulates. 



