66 THE NAUTILUS. 



lique parallelogram ; others, from Michigan and Minnesota, are 

 very high, the altitude equalling or even exceeding the length. 

 Some of these local forms may prove to be true varieties. 



This Pisidium has caused considerable trouble, correspondence 

 and controversy for a long time. Almost two years ago it was 

 recognized as a well-defined species, and given its present name. 

 Then Mr. E. W. Roper obtained a type specimen of Pi*, ferrugi- 

 neum Prime, from the Museum of the Boston Society of Natural 

 History, which he kindly sent me for comparison, and we were both 

 satisfied at once that it was identical with the present species. 

 Several examples, of T. Prime's own hand, also named P. ferrugi- 

 neum, from " New York," in my collection, probably none of them 

 mature, are of the same species. After this, the present name was 

 suppressed, although it was evident that all these Pisidia were very 

 far from being congruent, as to size and shape, with the author's de- 

 scription and figures of Pis. ferrugineum, in Mou. Pis. and Mon. 

 Corbiculadae. Among the thousands of specimens seen from New 

 England and New York, none could be referred to these descrip- 

 tions, and so necessarily the question arose : What, and where, is the 

 true P. ferrugineum of Prime 1 Last winter, Mr. Roper received sev- 

 eral lots of Pisidia from Cambridge and Waltham, Mass., and from 

 Maine, and obliged me by forwarding them for examination. 

 Among them there was undoubtedly the long sought for Pis. ferru- 

 gineum, in every particular conforming with the author's description 

 as well as the figures in Mon. Pis. (PI. XII, figs. 8, 9, 10). Now we 

 knew also that Pis. paupercHluin was distinct and deserving a name 

 of its own. The mixing up of the two species by Prime, is ex- 

 plained by the fact that both of them are usually covered with a 

 dark or blackish " ferruginous " substance, in the same way, giving 

 them the same outward appearance, the more so as in some forms or 

 specimens of Pis. paupercuhim the beaks are rather high and promi- 

 nent, though rounded, and not " tubercular," without ridges (Conf. 

 the figures cited above). Under the impression that they were iden- 

 tical, the author could say that P. ferrugineum was one of our most 

 common species, while properly restricted, it seems to be rather rare. 



Pisidium aoutellatum n. sp. 



Mussel of medium size, rather high, oblique, markedly protracted 

 downward in its anterior part, well rounded, rather strongly in- 



1 The author himself could not be consulted, since he had given up, long 

 ago, the study of these mussels. 



