1896.] NATURAL SCIENCES OF PHILADELPHIA. 569 



partial isolation. Further separation of the slough from the stream 

 is effected by the growth of willows and other vegetation upon the 

 alluvial ridge at its head, which rapidly gains in height by the debris 

 collected thereby. The lagoon of quiet water thus formed is a very 

 favorable station for molluscan and other aquatic life, sedentary 

 animals, or those of weak locomotive powers becoming far more 

 numerous than in the active current of the parent stream. Such a 

 lagoon thus gradually fills up with fine mud partly composed of or- 

 ganic material. In the case under consideration, the black clay 

 represents this period. During this time the mussels flourished in 

 the still water. Finally the lagoon or " slough " became dry land, 

 this being the ordinary result of the process. 



The naiad fauna of the Fish-house deposit is precisely similar in 

 general character to that of the " sloughs " of the Mississippi River 

 to-day. 



The cap of sand upon the black clay may be regarded as a later 

 deposition of more general geographic distribution, while the forma- 

 tions it overlies in this place are believed to be the result of strictly 

 local causes, and antedating by a lapse of time, greater or less in 

 duration, the overlying gravels. 



As to the fossils themselves, it must be admitted that their diver- 

 gence from living forms is very slight in most cases — a fact which 

 Dr. Lea significantly indicated by his choice of specific names. 

 Some of the species are really not distinguishable from modern shells. 

 Thus Unio nasutoides has no characters which can not be readily 

 paralleled in the living Unionasutus or fisherianus. Anodonta cor- 

 pulentoides is equally indistinguishable from A. corpulenta. The ab- 

 solute counterpart of Unio radiatoides may be selected from any 

 collection of U. radiatus, and so on. The remarkable feature of the 

 series of fossil forms is that certain of them have no modern repre- 

 sentatives in the Atlantic drainage south of the Great Lake and St. 

 Lawrence system. The following " species " exemplify this state- 

 ment : U. ligamentinoides, alatoides, prceanodontoides, rectoides, Ano- 

 donta grandioides and corpulentoides. Although the affinities of 

 some of these may have been wrongly estimated, owing to imperfec- 

 tion of the specimens, still a portion of them unquestionably bears 

 out the relationships affirmed by Dr. Lea. The majority of these 

 species foreign to the modern Atlantic drainage have their living 

 allies in, or are identical with, species of the Great Lake system, ex- 

 tending also into the northern Mississippi drainage in which, more- 



