338 THE PEARL Y FRESH- WA TER MUSSELS— SIMPSON. 



Atlantic, and the Flint River, which empties into the Gulf. The 

 great group of Unio buckleyi, which is so characteristic of Florida, 

 the coast region of (Jeorgia, and South Carolina, is so closely related to 

 that of Unto crassidens on the one hand and Unio complanatiis on the 

 otber, that the systematic i^ositiou of many of their species is wholly 

 uncertain. Again, the group of Unio Jisheri anus, also characteristic of 

 the Atlantic region, almost insensibly merges into that of U.lmcMeyi, 

 through such forms as U. ahenens, Lea, U. oscari, B. H. Wright, and 

 U.hazelhurstianits, liea; and the small group typified by U. doivniei, 

 Lea, inhabiting Georgia and Florida, shows about equal relationship to 

 those of U. crassiilens, U. hncldeyi, and U. comphmaius. 



In 18GS Lea described a number of fossil Kaiades^ from a marl bed uear 

 Camden, New Jersey. He believed this bed to belong to the Greensand 

 of the Cretaceous, and noticing the strong resemblance of the forms to 

 many now living in the United States, gave them names indicating 

 this resemblance. The age of these beds is uncertain, but is probably 

 more recent than what Lea supposed. The fossils are all casts of a 

 somewhat ferruginous marl, and are quite imperfect, but among them 

 are forms strikingly like Unio anodoiitoides, U. rectus, U. complanatiis, 

 and Anodonta corpulenta, and I think it not unlikely that they are in 

 most cases the remains of living species, and that the beds are not 

 older than the Pliocene. At any rate, they seem to show a much more 

 intimate mingling of Atlantic and Mississippi forms at the time they 

 lived than is now known to exist anywhere in either of the two regions. 



As 1 have sh iwn before, many of the species of the Mississippi Val- 

 ley extend into Canada; they occupy almost exclusively the southern 

 peninsula of Michigan, the Great Lakes, the Eed River of the North, 

 and the drainage system of the Mackenzie. This migration, which is 

 entirely distinct from the earlier mingling of eastern and western 

 species, is due, no doubt, entirely to the influence of the Glacial epoch. 

 It is now generally admitted that during this time a vast cap of ice 

 covered a greater or less extent of the Arctic and North Temperate 

 regions of North America, and that at the close of the Ice age the 

 southern edge of this cap gradually melted back for some distance 

 from its extreme limit. North of the Height of Land in British North 

 Ameiica great lakes were formed, which could only drain into the 

 Mississippi Valley, since the wall of ice on the north and east formed a 

 barrier in that direction. Several of these ancient drainage beds have 

 been discovered; one near Chicago, another at the western end of Lake 

 Superior, by which the water flowed down the St. Croix River; a third 

 down the Minnesota River by way of the Red River of the North, and 

 still another along the Maumee across to the Wabash.^ It is probable 

 there was an overflow down the Missouri River, as Unio margaritiferus 

 is found in the upper waters of this stream — the only point where it is 

 known to occur in the Mississippi basin. 



' Proc. Acad. Nat. Sci. Phila., XX, pp. 162-164, 1868. 

 - See Popular Science Monthly, XLVI, No. 2, p. 217. 



