MAZATLAN UNIVALVES 329 
377. CECUM REVERSUM, 7. s. 
C. (Fartulum) t. guniore “C. dextroverso” simiti, sed septo 
subconvexo, mucrone reverst, semi-cylindraced, arcuatd, aper- 
turam versus convert, dorsum versus concavd ; t.adultd ?....; 
operculo ?.... 
This species is constituted from a single young shell, in 
consequence of the extreme peculiarity of its plug. ‘This, 
instead of having the apex turned to the back of the shell, (or 
central as in the mammillated species,) has the highest part 
towards the front. It appears like a minute curled Cecum, 
lying on the slightly swollen surface of the plug, with its back 
towards the mouth, and its horns towards the back of the shell. 
It was at first regarded as a monstrosity of one of the other 
species ; but a careful examination under a high power shews 
it to be quite regularly formed. Long. ‘048, lat. *012. 
Hab.—Mazatlan ; 1 young sp. off Spondylus ; L’pool Col, 
Tablet 1549 contains the specimen. 
378. CCUM TERES, 7. 8. 
C. (Fartulum) t. elongatd, arcuatd, tereti, levi; longitudin- 
aliter interdum obsolete subangulatd ; albidd, sew hue et allue 
semicorned ; aperturd parum constrictd, dein minime reflexd, 
parum declivi ; septo mucronato ; mucronem versus parum 
ascendente margine subrecto ; mucrone ad fronten subelongata, 
ad latus angustd, parva, obtusd : operculo vix concavo. 
This species forms the transition from the smooth to the 
ribbed forms; haying the shape of Elephantulum, with the 
sculpture of Fartulum. The plug resembles that of C. farei- 
men, but the mucro is rather broader in the front view, though 
narrow at the side. The operculum, found in a young speci- 
men, can scarcely be called concave. The texture in the finest 
specimen is in concentric portions of dirty white or horny, 
irregularly mottled. Only 5 sp. were found, in different con- 
ditions. The smallest measures long. °045, lat. °012. 
The largest ey awuleks, 7. se (OLE ——Cehe 
Hab.—Mazatlan ; extremely rare, off Spondylus ; L’pool Col 
Tablet 1550 contains 3 sp. of different ages, the middle one 
possessing its operculum. | 
Tablet 1551 contains 3 pseudo-cecous bodies, inserted for 
comparison. The largest of these, measuring ‘045 across, was 
