8 F. Ameghino — Geology of Argentina. 



already tripled that number. These new discoveries, which enrich 

 palseontology so much, will be described in detail in special works ; 

 meanwhile it is of interest here to take a rapid glance at the 

 principal new types. 



The armoured edentates are abundant, but it is evident that the 

 representatives with a ringed carapace, which form the groups 

 of Dasypoda and Peltateloidea, predominate over those with a 

 more or less immovable carapace of the type of the Glyptodontia. 

 Moreover, the latter are only represented by species whose carapace 

 lacks the external sculpture, as in the more modern genera Neuryurus, 

 Fseudoeuryurus, etc., which, because of this peculiarity, Lydekker (in 

 his recent work on the fossil edentates of Argentina ^) considers to 

 have been founded on remains of young individuals in which the 

 sculpture has not yet been developed. According to this curious 

 interpretation, the Glyptodonts of the age of Pyrotherium, more 

 fortunate than ourselves, remained always young, since the sculpture 

 on the surface of the carapace was never developed. These ancient 

 armadillos, for which I have proposed the name of Palcsopeltis, 

 differ also from the most recent forms in exhibiting the plates in 

 well-defined transverse rows and to a great extent movable, thus 

 constituting a complete transition between the most modern Glypto- 

 donts with immovable carapace and the armadillos with ringed 

 carapace. 



The unarmoured edentates are also numerous and of types re- 

 sembling those of the Santa Cruz Formation, but generally of 

 much more considerable size. Nevertheless, some forms show very 

 primitive characters, having the molars provided with a well- 

 developed layer of enamel. 



With these edentates there are carnivorous animals of a size 

 approximating to that of the largest bears of the present day, but 

 similar to those of the Santa Cruz Formation, and thus belonging 

 also to the group of Sparassodonta, which is distinguislied by 

 a mixture of the characters of the polyprotodont Marsupials, the 

 Creodonta, and the placental Carnivora. 



The Plagiaulacoidea are also represented by various forms, of 

 which the multituberculate molars exhibit a remarkable transition 

 between the more modern types of the Santa Cruz Formation and 

 those of the Mesozoic formations of Europe and North America. 



The rodents are not numerous and appear to appi'oach the 

 Acaremyini of Santa Cruz, but they differ from these in exhibiting 

 five lower molars in function on each side of the mandible, two 

 premolars, and three true molai's. Except in the Leporidse, this is 

 the greatest number of molars met with in mammals of this oi'der : 

 that we are dealing with a primitive character is evident, and this 

 confirms my opinion that the hystricomorphous rodents originated 

 in South America, and not in the Old World as most authors 

 suppose. 



1 E. Lydekker, "Extinct Edentates of Argentina" : Anal. Mus. La Plata — Pal. 

 Argent., vol. iii, 1895, pt. 2. 



