444 The American Naturalist. 
mals did not make their appearance in the Argentine Repub- 
lic until the end of the Miocene. ; 
The new specimens of fossil rodents taken from the Santa- 
cruzian formation afford unexpected opportunities for com- 
parison. The fossil Cercolabinæ of Patagonia are the ancestors 
of all the hystricomorphous rodents. The Stiromys have, 
while young, five superior molars on each side, but the next to 
myinæ. The Eocardiinæ, in their turn, have given rise to the 
Caviidæ and to the Dasyproctinæ. The Octodontidæ, which 
now seem to be isolated, have come from certain forms of 
true origin of the Octodontidæ. There are still some inter- 
mediate forms wanting, but, no doubt, they will be found soon 
in the fossiliferous formation of the Paranà. 
Acaremys and Sciamys, from the lower Eocene of Patagonia. 
The Myomorphs are not derived from the Acaramyinæ, they 
having lost their premolars and undergone various modifica- 
tions in the conformation of the skull. If fossil rats have not 
yet been found in the Parana beds, it is on account of their 
fragile bones, but, in all probability, they will be found there 
some day. : - 
I now know a part of the dentition of the animal that I ha’ 
named Tidaeus—this name being preoccupied, I substitute : 
it, Mannodon. The Mannodon trisulcatus is the first gen 
Plagiaulacidæ of Patagonia which has the inferior molars 
structed on the same type as those of the Multituberculata. It 
1s a genus closely related to Neoplagiaulax and Ptilodus. 
_ Many of the types of the Plagiaulacidw present a featu 
that is strange to be found among mammals. The Epant 
thidee, for example, have the inferior incisors beveled on t 
