80 CALIFORNIA ACADEMY OF SCIENCES. 



of 15 to 20 per cent, in dimensions. The size of shell, 

 combined with "other signs of maturity" and the finding 

 of fry within them, have been considered the best proofs 

 of an adult condition, but we find that the latter is not 

 quite reliable. For instance, specimens like fig. 3 of S. 

 raymondi, and others smaller than figs. 21, 22 or 25 of the 

 same Eastern species, contain fry, the shells being imma- 

 ture. It appears probable that the shells may continue 

 to grow for some time while the fry are forming, and 

 only liberate them when quite mature. As to size, we 

 have remarked that there seems good reason to believe 

 that this maybe much' influenced by environment, and 

 cannot be considered proof of distinctness, the same law 

 being well known to apply to other fresh-water shells. 



The calycles, which form such a remarkable character 

 of this division of Sfhcerium that Prime proposed to make 

 a subgenus of it, give us great aid in determining ma- 

 turity, especially in the more convex forms. It appears 

 from the profile figures that the most marked external 

 character distinguishing young from adult shells is the 

 position of the calycles on the beaks, these being at first 

 everted, and becoming more inverted as the shell grows 

 larger, until they meet in the middle line. Thus it seems 

 that the shell fig. 10 must be more mature than fig. 3, and 

 the profile 14 has the beaks closer together than any 

 other, leaving no room for more inversion, and proving 

 maturity. Fig. 21 seems less mature by this character 

 than either 3 or 11, but illustrates most nearly the con- 

 vexity of the form. Considering the variability of all 

 fresh-water mollusca, and also the extent of variation ob- 

 served in different examples of many marine bivalves, 

 there is little doubt that Mr. Prime himself, if living, 

 would now combine many more of his nominal species 

 than he did in 1865. From the descriptions given by 



