108 THE NAUTILUS. 



variable as to size and shape. In the middle, above, there is 

 usually a rather deep emargination in the crest. 



The anterior left, C2, is nearly straight to slightly curved 

 at its base, triangular, directed obliquely backward, and bent 

 upward, with the apex pointed, or nearly so, and opposite the 

 emarginate middle of C3. From its aspect in lateral view it 

 appears to be massive, and must be examined from other 

 angles also in order to see its real shape. 04 is more or less 

 oblique, little projecting, generally rather short, and occa- 

 sionally vestigial. It is evidently of less consequence in the 

 mechanism of the hinge than its equivalent in Pisidium. 



It may be added that this combination of "teeth" with 

 their interlocking is quite an interesting object of study. 

 The primitive shape in the young nepionic mussel, especially 

 of C3, should be compared and then the gradual changes 

 followed up to the final configuration in the adult. 



In Sphffrium the shape of the cardinals is somewhat the 

 same, but they are comparatively smaller and plainer; C4 is 

 quite small, short and often rudimentary or wanting. With 

 Eupera, the hinge is generally slight, and the cardinals are 

 small and plain, and C4 is more often rudimentary or want- 

 ing than developed. 



The Lamina:. It is well to distinguish between a lamina 

 in toto and its cusp, or apex as it is also termed, that is a 

 more or less projecting part of its crest, usually pointed. 

 Many lamina have no cusps or only rudimentary. Quite 

 generally the laminar cusps of the left valve, all and pll, 

 fitting into the grooves, or fossae, between al and alii an- 

 teriorly, resp. pi and pill posteriorly, are projecting over 

 the median plane, or the level of the valve-edges, while the 

 right ones are not so, or only exceptionally and slightly, and 

 except in reversed hinges (q. v.). 



In Sphccrium, Musculium and Eupera, the posterior laminae 

 are longer than the anterior, but the latter, al and all, are 

 generally stouter and have well-formed, pointed cusps, and 

 so has alii, though it is usually quite small. Sphasria gen- 

 erally have a distal, rounded cusp on pll, and a slighter one 



