CHAP. X. UNIONIN^E. MYSCA. 277 



species are unusually long and narrow, while their in- 

 terior lustre is by far more brilliant and iridescent than 

 in either of the preceding groups. The cardinal teeth, 

 although in some respects like those of JEglia, neverthe- 

 less deserve much attention, because they not only differ 

 from those of Unio and JEglia, but bear a remote analogy 

 to IRIDINA. They are the most crenated of all those 

 in the UNIONIN^ precisely as those of Iridina are the 

 most crenated in the entire family : they are likewise the 

 most parallel to the anterior margin; and although in 

 general distinctly divided, they yet have a propensity, 

 even in individuals of the same species, to unite, and run, 

 as it were, into each other ; so that in some specimens 

 of our My sea ovata (Jig- 66.), these two teeth will be 



perfectly divided; while in others of a younger age, from 

 the same pond, the division will be only marked by an 

 indentation on the ridge, the groove itself being almost 

 obsolete. Now, this is precisely in unison with shells 

 intended to represent Iridina, where the lateral and 

 cardinal teeth are united into a crenated line. This vari- 

 ation in shells of the same species is not only perplexing 

 to young conchologists, but to more experienced ones : 

 in general, however, we may consider it as a rule that no 

 Unio belongs to this group, which has the cardinal teeth 

 thick and obtuse ; nor does it comprise any in which one 

 of the cardinal teeth is not immediately beneath the 

 bosses. These positive and negative characters render 

 the discrimination of Mysca comparatively easy. Very 

 T 3 



