114 THE NAUTILUS. 



The same cannot be said of Pisidium. In quite young 

 nepionic mussels the lamina? are about of the same shape in 

 the several species, but differences become manifest early, and 

 in the adolescent and mature are extreme. In some species 

 the laminae are at slight angles to the hinge-plate, in others 

 at strongly marked angles (conf. figs. 5, 4) ; in some they are 

 very slight, in others A^ery massive; the cusps may be little 

 marked or strongly so, may be at the distal ends of the 

 laminaa or near the center. In some they are very short at 

 the bases, very abrupt, spine-like, fig. 71, and there are 

 forms where a small cusp is about all that can be seen of a 

 lamina. 



The Hinge in Classification. 



Up to date, some authorities have insisted on treating Mus- 

 culium as a subgeuus of Sphfcrium. There is hardly a speci- 

 men which cannot be recognized as being of one genus or the 

 other, by its hinge ; but there are other features also proving 

 that Musculium is decidedly distinct. 



As to specific distinction in Spharium, we may simply re- 

 peat what has been said above. True, there are very con- 

 spicuous differences in regard to size, curvature and heavi- 

 ness of the hinges, as exemplified by sulcatum and stamineum 

 or solidulum. But they are more of quantity than quality, 

 so to say, and moreover, extreme forms of one species are 

 often more different from each other in this respect than 

 some manifestly distinct species are from others. Other fea- 

 tures of the mussels must be considered also, such as size, 

 shape, prominence of the beaks, surface sculpture etc. Though 

 outside of the subject under consideration, it may be added 

 that many species show considerable variation in these re- 

 spects also, and exact identification is in many instances 

 rather difficult. The same is generally true with Musculium. 



In Pisidium the hinge formation is much more diversified 

 than in the other genera, as has been pointed out, and is a 

 principal factor in distinguishing species. It has been pro- 

 posed to consider the hinges alone, excluding all other fea- 

 tures of the mussel as too variable, and consequently worth- 



