216 MAZATLAN UNIVALVES 
Cremides rugosa, H. & A. Ad. Gen. i. 446. 
+ Fissurella chlorotrema, Whe. Zeit. f. Mal. 1847, p. 186, no. 40. 
Fissurella humilis, Ake. loc. cit. no. 41, 
P+. viminea, Whe. Zeit. f. Mal. 1851, p. 36, no. 131 (non Roe.) 
= 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. I7l. 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 avery 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 alge, and seldom rubbed ; while 
F’. virescens is generally rubbed ; and only one (doubtful) speci- 
men was found with alge. 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. macrotrema, Sow. 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.* Ina 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, lat. °97, alt. 28. 
A broad specimen as ye S87 ok 
A conical specimen Woe 555) Oy! yy Se 
A flat sp. (shape resembling *Clypidella) a (leas Saat a By 
An elongated sp. ,, se tL OR “Gy. ete 
Holes, ‘08 by ‘06; 13 by °04. Div: 130°—160°. 
* If further 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. 
