THE NAUTILUS. 107 



terior valve margin (the "nymph") and may even appear 

 to be a downward continuation of it. 2 



BAUDON, 1857, p. 42, etc., states that P. amnicum has two 

 cardinals in the right valve "reunies par leur sommet," and 

 says the same of the two in the left, which are in fact sep- 

 arate from the uepionic stage. More surprising is the state- 

 ment that casertanum has three cardinals in each valve, due 

 to a misconception from insufficient examination and pre- 

 conceived ideas. The author evidently has never examined 

 young specimens and observed the devlopinent from the 

 primitive formation to the adult. 



CLESSIN, 1879, pp. 8, 9, and WESTERLUND, pp. 18, 19 divide 

 the Pisidia in three groups: Fluminina (P. amnicum) with 

 two cardinals in each valve, standing "side by side"; Rivu- 

 lina (type P. supimnn) has one cardinal in the right and two 

 in the left valve, also "side by side"; in Fossarina there is 

 one in the right valve and two in the left, "one behind the 

 other," which really means: one above, resp. below, the 

 other; just as in place of "side by side", above, it should 

 be: one behind, or in front of the other, if the parts of the 

 animal are considered (anterior and posterior, dorsal and 

 ventral ) . This by the way. But between the two last groups 

 there are intermediate forms, and it appears that they are 

 not clearly separable. 



In Musculium, the right cardinal is markedly different 

 from that of Pisidium and, it might be said, of a quite pecu- 

 liar formation (see pi. 3, fig. 13). Its anterior part may be 

 strictly longitudinal, straight, and rather long, but more often 

 it is oblique, more or less curved with the convexity below; 

 at the center it is strongly and sharply curved down with 

 C3i directed at right angles towards the plate edge, or more 

 often even forward, forming a hook, C3o is large and wing- 

 like, somewhat different with the several species, and rather 



2 The fact has often been overlooked that the dorsal shell margin of 

 each valve is not continuous but interrupted by the ligament. The 

 proximal, or central, end of the anterior part is slightly bent inward 

 under the anterior ' ' end ' ' the initial part of the ligament. 



