216 MAZATLAN UNIVALVES 



Cremides rugosa, H. 8f A. Ad. Gen. i. 446. 



+ Fissurella chlorotrema, MJce. Zeit.f. Hal 1847, p. 186, no. 40. 



Fissurella humilis, MJce. loc. cit. no. 41. 



? + F. viminea, MJce. Zeit.f. Mai. 1851, p. 36, no. 131 (non Rve.J 



= Fissurella virescens, jun. P. P. C. Cat. Prov. 



It is not to be wondered at that Menke did not find any 

 figure in the Conch. III. answering to his shells ; as the species 

 was described from not very characteristic specimens of a rare 

 and extreme form of this very variable shell. Although placed 

 by H. & A. Ad. in a different subgenus from F. virescens, 

 some forms run so closely into the young of that species, that 

 it was only after repeated examinations of some thousands of 

 specimens, and the fortunate discovery of a very few of the 

 real F. virescens, jun. that I felt justified in separating them. 

 The chief differences are, (1) that F. virescens is of a very 

 regular, F. rugosa very irregular habit of growth : (2) that 

 F. rugosa is much smaller, and never has the hole central, 

 though the amount of inequality is very variable : (3) that 

 the hole of F. virescens is always much larger in proportion. 

 The station also appears different, as F. rugosa is very fre- 

 quently entirely covered with algse, and seldom rubbed ; while 

 F. virescens is generally rubbed ; and only one (doubtful) speci- 

 men was found with algae. In the ordinary growth of the shell, 

 the ribs are not much more unequal than in F. virescens. The 

 most remarkable feature in the species is the great variation in 

 the shape of the hole ; in this respect resembling F. ornata, 

 Nutt. and F. macro trema, Soio. The extreme forms however 

 pass into each other by such insensible gradations, that it is 

 scarcely possible to regard them as otherwise than one species. 

 The youngest specimen, though only '035 in length, with a well 

 formed suboval hole, has no spiral vertex ; there is however a 

 scar where it may have adhered.* In a larger specimen, ' 11 in 

 length, the shell is formed as in the adult, light red, with an oval 

 hole. The largest specimen measures long. 1*45, tat. '97, alt. 28. 



A broad specimen 1*18, "87, '34. 



A conical specimen *92, '62, *3- 



A flat sp. (shape resembling Clypidella) '74, '53, 'II. 



An elongated sp. T03, *56> "24. 



Holes, '08 by '06 ; 13 by '04. Div. 130 160. 



* If furthfer investigation should prove that the apex of these species is decidu- 

 ous, while in the next section it is persistent until it becomes absorbed in the 

 progress of the hole, it will form a remarkable character of separation between 

 the groups. 



