98 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



corresponding to the similar area on the tarsal, as above described. A 

 very marked difference from Proterothermm is in the large plantar hook, 

 or beak, which in the latter genus projects very prominently backward, 

 but rises very little above the ectocuneiform facet and has no articulation 

 with the cuboid or ectocuneiform. In Thoatherhim, on the other hand, 

 the hook does not project so far backward, but rises far above the level of 

 the proximal articular surface and bears upon its fibular side a long, 

 narrow, oblique facet for the cuboid, and on the dorsal side is a very small, 

 but prominently projecting facet for the ectocuneiform. 



Metatarsal IV is a small nodule, tapering distally to a blunt point and 

 articulating by small, plane facets with the cuboid and mt. III. Ame- 

 ghino has figured a minute bone ('94^, p. 270, fig. ii^S") attached to the 

 fibular side of the cuboid, which he regards as a vestigial mt. V. In none 

 of the specimens which I have examined is this bone retained, nor is there 

 any facet for it upon the cuboid. I am therefore inclined to believe that 

 this bone cannot be regarded as a normal member of the metatarsus, 

 especially as nothing of the kind is indicated in any of the preceding 

 genera of the family. 



Phalanges are present only in the functional digit. The proximal 

 phalanx is longer, much more slender and less flattened planto-dorsally 

 than in Proterofheriujii ; the groove of the proximal trochlea for the meta- 

 tarsal carina is wider and has more sloping sides, and notches the dorsal 

 border somewhat more deeply : the distal articular surface is not reflected 

 so far upon the dorsal surface and narrows more proximally. 



The second phalanx is also narrow and less depressed than in Protero- 

 thermm; the proximal trochlea is more deeply concave and the median 

 dorsal beak is much more pronounced, while the distal trochlea is more 

 concave transversely. 



The ungual phalanx is longer, narrower, more pointed and much less 

 depressed than that of the manus, and consequently has a much more 

 convex dorsal surface ; the proximal trochlea is obscurely divided into two 

 condyles and has a smaller transverse and greater dorso-plantar diameter 

 than that of the manus. Curiously, the hoofs of Thoathernmi have a less 

 equine appearance than those of the tridactyl genera, which is perhaps to 

 be explained by the small size and lightness of the animal. 



On a superficial examination, the pes of Thoathermm appears to be 

 strikingly like that of the horse, but a closer study reveals a number of 



