434 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



The diagnostic features of the genus are to be found in the dentition, 

 which has a general resemblance to that of the recent Viscaccia (Lago- 

 stomus], but with well marked and characteristic differences. The incisors 

 have plane faces and, in comparison with the size of the animal, they are 

 quite slender and short, the upper ones extending but a little distance 

 into the maxillaries ; the lower incisors are longer and extend along the 

 inner side of the mandible to a distance which differs in the various spe- 

 cies, but never reaching behind the molar series. The grinding teeth are 

 entirely rootless and are composed of obliquely transverse 

 FIG. 40. laminae, much as in Viscaccia, with the difference that the 

 laminae are not in contact, but separated by a valley which, 

 at least in the larger species, is filled with cement; the 

 valleys open outward in the lower teeth and the upper pre- 

 molar, inward in the upper molars. In the lower teeth the 

 laminae are connected on the inner side by a narrow bridge 



Grinding teeth J 



of Viscacda chi- f enamel, and on the outer side in the upper molars, while 

 lensis. Crown in the upper premolar both enamel and dentine are con- 

 view, x \. a. tinued around the inner end of the valley, so that this tooth 



upper j.j as q u j te a diff er ent appearance from any of the others. 

 teeth, b. Right _, 

 lower teeth * ne tnir( i upper molar also differs from all the other grind- 



ing teeth in having three transverse laminae and two valleys, 

 the posterior lamina being shorter and more pyriform than the others. 

 In both jaws the two series of teeth converge strongly forward. 



The skull is much like that of Viscaccia in its general proportions and 

 in the shape and connections of its various elements, but there are certain 

 not unimportant differences. The occiput is broad and low, with nearly the 

 same relative height and width as in Viscaccia, but the bones are thicker 

 and the occiput is not plane, but has a marked median convexity, with a 

 prominent vertical ridge ; there is no such median fontanelle as is found 

 in the modern genus ; the foramen magnum is of a more transversely oval 

 shape and the condyles are somewhat larger. In none of the specimens 

 are the paroccipital processes preserved entire, but it is evident that they 

 were much shorter than in Viscaccia, though better developed than in the 

 other recent genera of the family. The supraoccipital has a somewhat 

 broader surface on the dorsal side of the cranium than in Viscaccia and 

 the occipital crest is correspondingly thicker; as in the latter, the bone 

 has long lateral processes, which extend down over the periotics, but are 



