18 THE NAUTILUS. 



rather than the exception in M. transversum Say. By the way, it 

 may be said that calyculate beaks are found occasionally in specimens 

 of Pisidium and Sphcerium. 



The nepionic (embryonic) mussel when discharged from the parent 

 is generally well inflated (except in il/. transversum), and then the 

 postembryonal part of each valve is marked off from it by a constric- 

 tion more or less deep. This seems to be especially well marked 

 when the embryos have been retained by the parent for a long time, 

 e. g., over winter, and are overgrown, as it were.^ Under favorable 

 conditions the embryos are probably discharged as soon as sufficiently 

 developed, moderately inflated, and then postembryonal growth goes 

 on in the same direction without or with a slight demarcation line. 



There are in my collection a number of lots of a MuscuUum, dif- 

 ferent from all other species described and known, from Rhode 

 Island, Virginia, Ohio, Michigan, and remarkably alike. The mus- 

 sels are somewhat like medium-sized M. securis Pr., but more elon- 

 gate, moderately and evenly inflated; the anterior and posterior parts 

 are less disproportionate, the latter is less high, less and more ob- 

 liquely truncate, the beaks are not calyculate, rounded, comparatively 

 broad and not very prominent; even under the microscope, no de- 

 marcation line between the embryonal and postembryonal parts can 

 be seen; the surface is markedly regular, without or with slight lines 

 of growth, with very fine strise and a slight silky gloss; the color is 

 dark horn, not yellowish, somewhat lighter along the margins, but 

 there are no sharply defined zones, as common in securis. Isolated, 

 this Musculium would appear to represent a distinct species; but 

 younger specimens, evidently of the same form, have more the out- 

 lines of ilf. securis, and in every lot there are some specimens of the 

 same, with the beaks calyculate, and generally there are intermedi- 

 ate ones, as to outlines and general appearance. Several of the lots 

 were collected in fall, from September to November, and others 

 probably so. It appears probable that this is a summer form of 

 M. securis, of fast and steady growth under favorable conditions, 

 consequently not a variety. Corresponding forms of other species 

 have also been seen. If verified by future observations, this is a 

 remarkable and very interesting fact. 



Also the varieties and local forms of all species, and the conditions 



> Under the microscope such specimens show several concentric zones along 

 the margins of the valves, marked by lines of growth. 



