PREVIOUS TO THE LAST GEOLOGICAL REVOLUTION. 389 



mands a supplementary notice. T have before mentioned 

 that of each of the genera Cictia, Paca and Capivar, only 

 a single species now exists in this district, whereas I have 

 found two fossil species of Cutia and Capivar, and three of 

 Paca ; that of each of these genera one species is of gigantic 

 proportions, while the remainder correspond in size to the 

 existing species. Again, I have there observed that of the 

 two lesser species of Paca\ one resembles the living animal 

 in its nearly smooth head, (although in the structure of its 

 cranium it otherwise displays sufficient specific difference,) 

 while the other species is distinguished by an extraordinary 

 development of its zygomatic arches, as likewise by great 

 inequalities on its cranium. I have at this moment received 

 the cranium of a Paca, shot near Curvello, that exhibits all 

 these characters of the fossil species, although a closer com- 

 parison convinces me also, in this instance, of their specific 

 distinction. I have quite satisfied myself that this develop- 

 ment of the zygomatic arches and unevenness of the surface 

 of the cranium are not the effect of age, by the examination 

 of a connected suite of the smooth-headed Paca from the 

 earliest to the most advanced age, in which all the sutures 

 have disappeared ; whereas the cranium here spoken of be- 

 longs to a young animal, in which the sutures are still di- 

 stinct. Instead of this, I have reason to suspect, however 

 little such a phsenomenon may appear to be supported by 

 analogy, that this striking dissimilarity in the formation of 

 the skull results only from a difference of sex. I rest this 

 supposition on two principal facts ; first, because, with this 

 single exception of the crania, I do not find in all the other 

 bones belonging to the Paca the least dissimilarity to be- 

 token the existence of two distinct species ; and, secondly, 

 because the characters by which the fossil smooth-headed 

 Paca is distinguished from the living, display a remarkable 

 parallelism with the distinctions that characterize the rough- 

 headed Paca of the same two periods. 



Should this supposition be confirmed, then the two species 

 that I have described under the names of Coelogenys rugiceps 

 and C laticeps, must be reduced to one, which may preserve 

 the name laticeps, and the genus Paca will thus possess the 

 same proportions as the genera Cutia and Capivar. 



^ Dr. Lund does not appear aware that there are now existing in South 

 America two species or varieties of Paca (Coelogenys), the skulls of which 

 differ precisely as above described. These differences were first pointed out 

 by Cuvier in the * Annales du Museum/ tom. x. p. 203. pi. x. 



