PROCEEDINGS OF THE NATIONAL MUSEUM. 



333 



OH the other side. I do not know tbat any other group of Knios is rep- 

 resented on the western sh)pe, and so far as 1 am aware, none of the 

 other genera have as yet been met with in the rather hiuited drainage 

 of that region. 



The great group of Glabaris typified by (t. tntpe.zialis, Lamarck, a 

 very natural and closely related assemblage, is well represented, no 

 doubt, throughout all the eastern and soutlieastern drainage of South 

 America, from well down in Argentina to Central America and even 

 southern Mexico. Indeed, the typical species is in the Museum collec- 

 tion from the streams of Argentina to Lake Maynos in the interior of 

 Peru, the San Francisco River, Brazil, and the Rio Xegro on the north. 

 The group is well represented in Central America and southern ^lexico 

 by G. hridgesi. Lea, and allied forms. A single species, G. Jeotandi, 

 Guppy, is found in Trinidad, ^o species and only three or four groups 

 of this region are extralimital.' 



The Central American Region. — All Central America, including, jjer- 

 haps, the most of the Isthmus of Panama, and all of Mexico excejit the 

 strip west of the Cordillera, together with Yucatan and the Island of 

 Cuba, form a single Naiad province which is jDeopled with a, large number 

 of Uiiios, a fair representation of Anodontas, a single Mycetopoda^ and 

 a few Glabaris. The fauna consists really of three elements, which no 

 doubt represent as many migrations. 



First. — A large number of ITnios, constituting the greater part of the 

 fauna, which by their solid, sometimes angular and inflated forms and 

 often pustnlous or somewhat jilicate sculpture, indicate evident rela- 

 tionship to groups in the Mississippi Valley. The groups showing these 

 resemblances are placed opposite each other in the following table: 



Relationship of Central American and Mississippi Valley Unios. 



The group of Central American Unios, typified by U. aratus, does not 

 seem to have a parallel in any assemblage of Mississippi Valley forms, 

 but is undoubtedly related in a general way. The Unios of this region 



'Attention may be called to the curious fact that a number of the South American 

 species of Unios are imitated by certain Glabaris which very strongly resemble them 

 externally. Thus the orbicular Unios typified by U. iiocturnns have their parallel in 

 Glabaris in a section typified by G. lato-marginafa, and the elliptical Unios of the 

 Casablanca group are balanced by G. pitelchana, etc. Unto delodoutus and its allies 

 are ofi'set by G. wymani and others, and the elongated solid U. parallelopipedon and a 

 few others have their counterpart in G. ensiformis, which sometimes so closely resem- 

 bles the members of this group that anyone would at once place it with tlieiu unless 

 the hinge was examined. There is no relation whatever between the genera. Their 

 resemblances are probably adaptive. 



