516 PATAGONIAN EXPEDITIONS I ZOOLOGY. 



lution of distinct species. (3) Ignorance of the Patagonian faunas be- 

 tween the Rio Negro and the Rio Santa Cruz system, and along the base 

 of the Andes, where Transition forms and intermingling of the species 

 may occur. 



GASTROPODA. 



Family ENDODONTIDsE. 



Two genera of this family are known from Patagonia : Stephanoda and 

 Radiodiscus. The relationships of these forms to genera of other regions 

 are unknown, since we have as yet no knowledge of their soft anatomy. 

 Endodontidce were present in the North American Carboniferous, repre- 

 sented by forms resembling the modern Gonyodiscus and Charopa in shell 

 characters, and like their descendants, living in and upon decaying stumps. 

 The family is now found all over the world, even on the most remote 



islands. 



RADIODISCUS Pilsbry. 



Radiodiscus Pils., Proc. A. N. S. Phila., 1906, p. 154, for R. milleco status. 



Minute, discoidal, openly umbilicate Patuloid snails, with the embryonic 

 \Y^ whorls minutely engraved spirally, the rest of the shell densely radi- 

 ally costulate. Aperture rotund-lunate, but slightly oblique, and as high 

 as wide. Type R. millecostatus Pils. & Ferr. 



In the Endodontidce, where small differences in the shell characterize 

 extensive series of species, it seems desirable to recognize as of generic 

 rank such readily definable groups as Radiodiscus. 



The distribution of this genus is very wide, extending from Tierra del 

 Fuego to the mountain ranges of the southern boundary of Arizona, where 

 it meets the Holarctic Gonyodiscus and the Nearctic Helicodiscus, both at 

 their southern limits. At present, the known distribution of Radiodiscus 

 is markedly discontinuous, one area extending from southern Arizona to 

 central Mexico, the other from southern Brazil to Cape Horn ; yet it must 

 be remembered that the Andes and northern South America are un- 

 searched for minute shells. We know very few so small as these (2 to 

 3 mm. diam.); and some of the species imperfectly described may turn 

 out to belong to Radiodiscus. It is likely that the group is an Austral 

 one, which has invaded Mexico from the south. 



Some Tasmanian snails have a great resemblance to Radiodiscus, in 



