NO. 2291. MAMMALIAN AND FISH REMAINS FROM FLORIDA— HAT. 105 



the bone. Owen writes^ that the anterior and inferior surface of the 

 astragalus of Mylodon rohustus is occupied by one extensive elongated 

 articular surface adapted to the calcaneum, cuboides, and navicu- 

 lare, and his figure shows that this is true. On the other hand, in 

 the bone from Florida, this surface is completely divided by a deep 

 rough furrow for ligaments. It will be noted, too, that the out- 

 lines of the two bones as seen from below are very different. 



It might be supposed that the astragalus here described is that of 

 Megdlonyx; but this bone was described by Leidy,^ and only a glance 

 at Leidy's figures is needed to convince one that the Florida bone 

 can not belong to that genus. 



Owen^ described and figured an astragalus which had been brought 

 from South America and which he thought belonged possibly to 

 Megalonyx. This was afterwards made by Leidy * the type of a new 

 genus and species, GnatJiopsis oweni. When the Florida bone is 

 compared with Owen's figures here reproduced (pi. 27, figs. 3, 4) 

 there are to be seen close resemblances. It might not be far out of 

 the way to refer the astragalus from Florida to a second species of 

 Gnathopsis, but a careful examination shows differences that seem 

 to indicate a distinct but closely related genus. On the upper 

 surface of the bone figured by Owen there was certainly no such 

 deep fossa for a process of the tibia and for a ligament as is seen in 

 the Florida bone. Nor was the surface for the tibia as convex as 

 it was in the bone here described. Again, as seen from below, there 

 was in the astragalus of Gnathopsis a deep and wide notch in the 

 anterior border at the end of the ligamentous groove, as if this and 

 the upper one joined across the border of the bone. In the Florida 

 bone the anterior surface for articulation with the calcaneum extends 

 nearly to the inner border of the bone; in Gnathopsis oweni it is 

 much shorter. Believing that such differences in as characteristic 

 a bone as the astragalus is among the ground sloths, indicate other 

 important differences in the skeleton, the name TMnohadistes is 

 proposed for the genus, the species to be known as Thinohadistes 

 segnis. (Derivations, Ols sand; ^ahcT-qs, a walker; segnis sluggish.) 



The following measurements in millimeters have been made on 

 the astragalus here described: 



Extreme length of astragalus 108 



Width from summit of tuberosity for tibia to border between the 



fibular and the calcaneal surfaces 97 



Length of surface for tibia 76 



Width of surface for tibia 70 



Height and length of surface for fibula 38 



Length of posterior articular surface for calcaneum 69 



Width of posterior articular surface for calcaneum 38 



I Descr.skel. Mylodon rohustus, "p. 118. 



« Smiths. Contrib. Know!., vol. 7, art. 5, p. 40, pi. 12, figs. 7-10. 



' Descr.skel. Mylodon rohustus, p. 132, pi. 23, figs. 3, 4. 



* Smiths. Contrib. Knowl., vol. 7, p. 41. 



