32fi MAZATLAN UNIVALVES 



in C. undatum. About 170 specimens were found (of which at 

 least 41 possessed their opercula) agreeing in the above dis- 

 tinctive characters. The texture of the shells, when fresh, ia 

 not alike in the two forms ; C. undatum being of a somewhat 

 waxen white, or alabaster : C. lajve being of a more horny 

 Colour, occasionally developing a marked brown, especially 

 withm, 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 — '01. 



An unusually large one ,, "07 „ '018. 



Sab. — Panama ; extremely rare ; C. B. Adams. — Mazatlan ; 



not uucommon, on Spondylus, Chama, &c. ; L'pool Col. 



Tablet 1539 contains 6 sp. very young, of dijBferent curvatures 

 and proportions. — 1540, 6 sp. 'monstrosum' period, do. — 1541, 

 4 sp. 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. — 154:B, 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. abnonnale, p. 31 (j.) 



373. CiECUM FAECIMEN, n. X. 



C. (Fartulum) t. " C. hvvi' simili, .fed alhida, aperturd minus 

 contracta ; t. adultd et hderdiim adolesceiite, annulis rotundatis 

 paucis suhohsoletis aperturam plerwmque prcecedentibm ; septo 

 plerumque hand elevato, apice laterali subprominente haud acuta, 

 utraque parte angusta; ad frontem mucronatd, ad latus mar- 

 glne parum ascendcnie, haud rectd ; opereulo concavo. 



This species differs from C. la^vc, 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 

 tilowl}', 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 



