326 MAZATLAN UNIVALVES 
in C.undatum. About 170 specimens were found (of which at 
least 44 possessed their opercula) agreeing in the above dis- 
tinctive characters. The texture of the shells, when fresh, is 
not alike in the two forms; C. undatum being of a somewhat 
waxen white, or alabaster: C. leve being of a more horny 
colour, occasionally developing a marked brown, especially 
within, where it frequently leads to delusive hopes of finding 
the operculum. ‘This is of the same size and concavity as in 
C. undatum; but in the specimen examined within does not 
possess the sutural line, and the whirls are apparently fewer. 
The smallest specimen measures long. °035, lat. ‘005—O1. 
An unusually large one aie 0/4 POTS! 
Hab.— Panama; extremely rare; C. B. Adams.—Mazatlan ; 
not uncommon, on Spondylus, Chama, &c.; ZL’ pool Col. 
Tablet 1539 contains 6 sp. very young, of different curvatures 
and proportions.—1540, 6 sp. ‘monstrosum’ period, do:—1541, 
4sp.adolescent. Of these two have the profile line of the plug 
somewhat swollen, and one is more tapering than usual. The 
latter may be the young of some other species.—1542, 6 sp. 
adult, varying in colour, arcuation, &c. One of them is curi- 
ously and beautifully encrusted; and another retains part of 
the decollated portion round the plug.—1543, 6 sp. of different 
ages with their opercula. One is of deformed growth: another, 
of mature breadth, is so short that it would have been consider- 
ed broken were it not for the operculum. This seems to shew 
that the animal is not necessarily as long as the persistent shell. 
(Comp. C. abnormaie, p. 316.) 
373. CHECUM FABRCIMEN, 2. S. 
CO. (Fartulum) t. °C. levi” simili, sed albida, aperturdé minus 
contracté ; t. adulté et interdwin adolescente, annulis rotundatis 
paucis subobsoletis aperturam plerwnque precedentibus ; septo 
plerumgue haud elevato, apice laterals subprominente haud acuto, 
uirdque parte angusta; ad froniem mucronata, ad latus mar- 
gine parum ascendente, haud recta ; opereulo concavo. 
This species differs from C. leve, which it is very much like, 
in shape, size and general appearance, in the form of the plug 
which closely resembles that of C. firmatum. It rises very 
slowly, and in a somewhat swollen line, to a distinct lateral 
knob which is short and narrow. The shell generally (but not 
always) develops obscure rings near the aperture. In the only 
