MAZATLAN UNIVALVES 327 
specimen found in the adolescent state, these ribs remain 
between smooth portions, shewing the position of a former 
mouth. An immature specimen displays, under a high power, 
extremely minute longitudinal irregular wrinkled strie, which 
are probably due to the epidermis. One operculum was found 
in situ, which appears somewhat less concave than in C. leve, 
but it was not extracted. An unusually large sp. measures 
long. ‘064, lat. °019. 
Hah.—Mazatlan ; very rare, off Spondylus, &e.; L’pool Col. 
Tablet 1544 contains 4 sp.; viz. 1 adolescent, 1 immature, 
and 2 adult, one slender, the other swollen, with operculum. : 
Tablet 1545 contains a sp. probably belonging to this species, 
with the plug abnormally swollen and the knob divided. 
374. CHCUM GLABRIFORME, 7%. s. 
C. (Fartulum) t. “C. glabro” simillimt, sed septo mamillato 
twmentiore, fere hemispherico ; operculo ?.... 
One young and four adult specimens were found (with 
fragments) ; but all were dead, and the operculum was not to 
be seen. It is almost exactly like the British species, differing 
in being a trifle broader, and in having the regularly hemi- 
spherical plug much more swollen, as in the young of C. gla- 
brum. The mouth is sharp, and scarcely contracted. It would 
have been curious to see whether the quoted convex operculum 
of C. glabrum finds its analogue in this species. Long. ‘073, 
lat. OVE: 
Hab.—Mazatlan ; extremely rare, off Chama; L’pool Col. 
Tablet 1546 contains 2 sp., young and adult, the latter, 
though broken, being the most characteristic in the prominence 
of the septum. In the British specimens, as well as in those 
from Mazatlan, the amount of tumidity varies not inconsider- 
ably ; so that the most prominent British shell scarcely differs 
from the least prominent Mexican. If it were not for the 
uncertainty of the operculum, added to the remoteness of 
habitat, the species might be united. 
3/75. CHCUM CORRUGULATUM, 2. s. 
C. (Fartulum) t. “C. glabriformi” simili, sed tumidiore ; 
superficie tenuissime transversim striata, striulis minimis irregu- 
lariter corrugatd ; septo ?’mammillato ; operculo ?.... 
