1895. PROCEEDINGS OF THE NATIONAL MUSEUM. SSI 



of this region does not seem to support the idea of any separation of 

 Mexico from the rest of North America during Tertiary time, but I 

 simply give what aj^pears to be the evidence of the Naiades.' 



It is possible that at some time during the occupation of this region 

 by the older Naiad tauna there may have been a strait through the 

 present Isthmus of Tehuantepec, which separated Central America 

 from Mexico. The strongly sulcate sculpture of most of the Unios of 

 this lower region may have developed under insular conditions, or it is 

 possible that it is wholly due to climate. 



The Naiad fauna of the Atlantic area, while consisting generally of 

 moderate-sized Unios and Anodontas, without, as a rule, any striking 

 characters, was, I believe, developed from that of the Mississippi Valley, 

 but it has long been separated by the Appalachian chain. Unio {Ano- 

 <Io)ita) undulatus, Say, of the Northeastern States, is only a mere variety 

 at best of the U, {Anodonta) edentulus, Say, of the Mississippi drainage. 

 lido {Margaritana) marginatus, Say, found in the former region, though 

 smaller and more delicate, is identical with the western species. Unia 

 radiatus of New England belongs to the western Lufeolus group, and 

 in some cases approaches so close to the type that the two cannot be 

 satisfactorily separated. Unio ochraceus, Say, and U. cariostiSj Say, 

 belong to the Mississippi group of U. ventricosus, while the groups of Unio 

 {Margaritana) calceolus and Unio pressus are about equally developed 

 in the two regions. The migration of these forms has no doubt been 

 made around to the northward of the Appalachian chain, as the species 

 belonging to these groups in the Atlantic drainage are abundant in 

 New England, but gradually vanish as we go southward. South of 

 the dividing range, the relationship is still more apparent. The great 

 Mississippi Valley groups of Unio tetralasmus, U. snbrofitratus, U. 

 crassidens, U. parvus, and U. ventricosus are all well represented in the 

 Atlantic drainage of Georgia, Florida, and in some cases as far north 

 as North Carolina, though there seems to be a slight separation of 

 the two areas between the Ocmulgee Eiver, which drains into the 



'I quote friun a letter received from Mr. H. A. Pilsbry, regarding the evidence of 

 the land and fresh-water snails in this connection: "Now as to Mexico, we have 

 there in the south a 'tincture' of South American types, evidently of recent origin. 

 The Solariopsis and Ldbyrinthus very likely came north in or since the Pliocene eleva- 

 tion of the isthmus. The Melanians of Mexico are distinctly South American. 

 Besides, Mexico has in the Eucalodium, Holospira, Glandina, etc., element a distinct 

 fauna, suggesting insular conditions both from the West Indies and North America, but 

 nearer the former. At all events, it looks as if the fauna of northern Texas and 

 New Mexico is a recent mingling of the two faunas, the Polygyras moving south, 

 and Holospira, Bidimulus, etc., moving north. How much this appearance is due to 

 mere isotherms, I am not prepared to say; but still, without having any tabula- 

 tion of the faunas before me, it looks as if to a peculiar nucleus of genera which 

 evolved their differential features on Mexican soil had been added lately au ele- 

 ment from South America, another from the West Indies, a third from the United 

 States, these introduced factors being still far stronger toward their respective 

 points of ingress." 



Proc. N. M. 95 22 



