EEPOET OF THE SECEETART. 23 



Many of them arc also furnished with so few positive specific external 

 characters-, that for their determination an anatomical difference in 

 their soft parts can alone be depended upon. To study, therefore, 

 satisfactorily single species of fluviatile shells, one must have before 

 him, says Mr. Binney, a very large suite of specimens of all ages, from 

 ever}^ portion of the district which it inhabits, as well as authentic speci- 

 mens of every allied described species, with an equally complete set 

 of individimls. From these facts it is evident that among the descrip- 

 tions of these shells which have been made by different authors, and 

 published in different works, there must be many which refer to the 

 same species; or, in other words, that the bibliography of this branch 

 of natural history must abound in synonyms exceedingly perplexing 

 to the student, as well as exhibit imperfections in the systematic 

 exposition of the subject which ought not to exist. Such a con- 

 dition must occur from time to time in every branch of natural 

 histor}^, and it therefore becomes important that some person com- 

 petent to the task should go over the whole field and reduce to what 

 is called a monograph, or single sketch, all that has been previously 

 done in regard to it. 



Mr. Binney in his portion of the catalogue does not attempt to pre- 

 sent a complete monograph, but rather a report on our present state 

 of knowledge relative to fluviatile univalves. He has given an Eng- 

 lish translation of all the original descriptions and a facsimile of the 

 outline of each original figure. The portion of this w^ork which re- 

 lates to the fresh-water univalves of North America intrusted to Mr. 

 Binney has been completed, and is now in the press. The remainder 

 on the Ilelaidadae, undertaken by Mr. Tryon, has been commenced; 

 but since the family contains a large number of species exhibiting 

 many of the variations above-mentioned, a year or more will be re- 

 quired before it can be completed. 



The ninth article, or that relating to the C/jcIadidae, is nearl}' ready 

 for the press. The recent species of this group are generally small 

 and inhabit the fresh waters of various parts of the world, although 

 some belong to brackish or even marine localities. Mr. Prime has 

 paid particular attention to this group, and is said to be one of the 

 first living authorities in regard to the subject. 



The eleventh article of the above-mentioned series wnll include 

 detailed descriptions by Mr. Stimpson of all the marine shells of the 

 eastern coast of the UnitedStates, with original notices of the internal 

 stracture. 



