278 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 



MEGALONYCHOTHERIUM gen. nov. 



(Plate XLVI.) 



This genus is represented by a single skull, without mandible (No. 

 15,593) fr m which unfortunately, all the teeth are missing, except the 

 basal portion of ^ on the right side. In general character the skull is 

 much like that of Hafialops, but the resemblance to Megalonyx is so much 

 more striking than in the case of any other known Santa Cruz fossil, that 

 it seems advisable to separate it generically from the former. 



The dentition has a very suggestive resemblance to that of Megalonyx, 

 as is evident from the empty alveoli ; - is much larger than in Hapalops 

 and of an entirely different shape, obviously resembling the same tooth 

 in Megalonyx, being laterally compressed and antero-posteriorly elongate, 

 but is more trihedral and less oval than in the Pleistocene genus, with the 

 anterior surface a rounded edge and the posterior relatively broad ; the 

 tooth is placed as far forward as possible in the maxillary, which has no 

 plate-like extension in front of it, such as occurs in Hapalops, and no 

 such rounded, pillar-like expansion as is found in Eucholceofis, for it does 

 not project outside of the line of the other teeth, as it does in the latter. 

 The first molariform tooth, a , which is separated by a considerable dia- 

 stema from -, is similar in form to that of Megalonyx, but relatively 

 larger ; ^ and - are of nearly equal size and resemble those of the Pleis- 

 tocene genus, while A is proportionately smaller than in the latter. 



In the skull also there is a suggestive resemblance to Megalonyx, with 

 many differences in matters of detail. In general proportions the follow- 

 ing differences from Megalonyx jeffersoni may be noted : The cranium is 

 relatively shorter and the face longer, but, at the same time, the diastema 

 is shorter ; these differences are largely due to a shifting of the orbits in 

 the Pleistocene species, in which the front rim of the orbit is well in 

 advance of 2 -, making the preorbital region extremely short, while in the 

 Santa Cruz genus the edge of the orbit is over the space between - and ^ ; 

 the whole skull, especially the facial region, is much shallower dorso- 

 ventrally ; the postorbital constriction is shallower and more anterior in 

 position, while the preorbital fossae are much more distinctly defined ; the 

 zygomatic process of the squamosal is more slender and much more 

 horizontal in direction. The sagittal and occipital crests are quite promi- 

 nent, the former more so than in any other known genus of the Santa 

 Cruz Gravigrada, and the muzzle is broad and abruptly truncate. 



