— '>21 — 



Unios well represented in Central America. Unio ore- 

 g-onensis Lea is allied to U. Rowellii Lea and others: 

 and I have no doubt that systematically organised re- 

 searches will learn us some more species of Unio of the 

 Pacific slope. The depauperation of Unionids is howewer 

 a very great and singular one, and we may be very 

 anxious to know the fossil species of Unio. 



In Mexico there is a singular aggregation of north 

 and central american Unionidae besides especial forms 

 and some south american ones. There existe Anodontas 

 (An. globosa Lea, nopalatensis Reeve, viridans Cless.) 

 besides of Glabaris (Gl. glauca Val, ciconia Gould, Brid- 

 gesii Lea, Strebeli Lea). (1) 



Thus the pacific province and the mexican are im- 

 portant ones, being on the contrary the Alleghanies a 

 barrier of secondary value in a comm m great province 



We must suppose, that the Alleghanies, althoug a 

 very ancient vrange of mountains, have ondergone some 

 changes of nivel and of hydrography during the ter- 

 tiary period, and that thus the rivers now separated 

 once were comunicating. Sooner, etablished the sepa- 

 ration hydrographie, some species remained conserved 

 identic, others remained on one side of the Alleghanies 

 extinguishing on the other, and other species have 

 ondergone modifications, which determine a specific 

 separation, as p. expl 



Atlantic sloi)e ^lùsisip^n drainage 



U. nasutus Say — U. subrostratus Say 



U. cariosus Say — U. ovatus Say 



U. ochraceus Say — U. multiradiatus Lea 



U. radiatus Lam. — U. luteolus Lam. 



U. complanatus Sol. — U. camptodon Say 



'^2) Next year I will publish a Catalogue of Glabaris. Gon- 

 chologicaly are the beaks (smooth in Glabaris, undulous etc., in 

 Anodonta) and the ligamental sinus quite sufficient for the 

 distinction of the two srenera. 



