136 THE .NAUTILUS. 



ists, however, more attention has beeii given in this direction, and 

 while perhaps some have gone to the other extreme and have 

 accorded too great importance to the hinge characters, there can be 

 no doubt, but that a careful and conservative study of our Amer- 

 ican species in this regard would be in many cases a valuable assist- 

 ance to the determination of the relations and specific validity of 

 many of the at present poorly defined and ill-understood forms. 



In examining some specimens of the common Sphaerium striatln inn 

 Lam. recently, I was struck in several instances with the occurrence 

 of an abnormal arrangement of the hinge teeth, and was thereby 

 induced to go further and ascertain if possible to what extent such 

 irregularities were present. 



My observations thus far have been confined to two species, 

 Sphcerium striatinum Lam., and S. simile Say. Of the former 

 species one hundred specimens from twenty-four localities, were 

 chosen at haphazard, the number from each locality varying from 

 one to eleven. Of these, all the specimens from sixteen local- 

 ities, forty-six in number, were normal. Of the remaining fifty-four 

 specimens from eight localities, ten were abnormal. In only one 

 instance, did more than a single abnormal individual occur in any 

 one locality, and in that, three out of nine specimens were abnormal 

 and all in the same particular. 



Of Spluerium simile Say, sixty-four specimens from nineteen 

 localities were examined, the number from each locality varying 

 from one to six. Of these, ten individuals out of thirty from eight 

 different localities were abnormal. While in those from eleven 

 localities, represented by thirty-four specimens, no variation occurred 

 In both species the number of abnormal specimens seems surprisingly 

 large, amounting to ten per cent in S. striatinum and to nearly 

 sixteen per cent in S. ximilc. In no case was any attempt made to 

 pick out unusual specimens and, indeed, no external variation in ap- 

 pearance was noticed as being coincident with the abnormal arrange- 

 ment of the hinge. 



The normal arrangement of the hinge-teeth of a Sphserium is as 

 follows : 



Right valve Cardinal 1 Ant. laterals 2 Post. Intends 2. 

 Left valve Cardinals 2 Ant. lateral 1 Post, lateral 1. 



The variations noticed consisted in the reversing of the position of 

 one or more pairs of these teeth. 



