MAZATLAH UNIVALVES 329 



377. CiECUM EEVEBSTJII, 71. S. 



C. (Fartuluni) t. juniore " C. dextroverso" simili, sed septo 

 subconvexo, mucrone reversd, semi-cylindraced, arcuaia, aper- 

 turam versus convexa, dorsum versus concavd ; t. adultd ?....; 

 operculo ? . . . . 



This species is constituted from a single young shell, in 

 consequence of the exti'eme peculiarity of its plug. This, 

 instead of having the apex turned to the bade of the shell, (or 

 central as in the mammiUated species,) has the highest part 

 towards the front. It appears like a mii\ute curled Carjum, 

 lying on the shghtly svroUen 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 xmder a high power shews 

 it to be quite regularly formed. Lonff. "018, lat. "012. 



Sab. — Mazatlau ; 1 young sp. off Spondylus ; L'pool Col, 



Tablet 1549 contains the specimen. 



378. C^CUM TEE.ES, n. s. 



C. (Fartuliim) t. elongatd, arcuatd, tereti, Icevi ; loiigitudin- 

 aliter interdum obsolete subanyulatd ; albidu, seic hue et illuc 

 semicortied ; aperturd paricm coiistrictd, deiti minime reflexd, 

 paricm declivi ; septo mucronato ; mucronem versus parum 

 ascendente margine suhrcclo ; mucrone ad frontern, subelongata, 

 ad, latus angustd, parvd, obtusd : operculo vix concavo. 



This species forms the transition from the smooth to the 

 ribbed forms; having the shape of Elephantulum, with the 

 sculpture of Fartulum. The plug reseiubles that of C. farci- 

 men, but the mucro is rather broader in the front view, though 

 narrow at the side. The opei'culum, 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. '015, lat. "012. 



The largest „ „ -115, „ -017—027. 



Sab. — 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-cajcous bodies, inserted for 

 comparison. The largest of these, measuring '015 across, was 



