MAZATLAN UNIVALVES 320 



377. CAECUM EEVEESUM, n. S. 



C. (Fartulum) t. junior e " C. dcxtroverso" simili, scd septo 

 subconvexo, mucrone reversd, scmi-cylindraced, arcuaiCi, aper- 

 turam versus convexd, dor sum versus concavd ; t. adult A ?....; 

 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 hack of the shell, (or 

 central as in the mammillatcd species,) has the bigl part 

 towards the front. It appears like a minute curled Csr cum, 

 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 shows 

 it to l}e quite regularly formed. Long. '048, lat. '012. 

 Jlab. Mazatlan ; 1 young sp. off Spondylus ; IS pool Col. 



Tablet 1549 contains the specimen. 



378. CAECUM TEEES, n. S. 



C. (Fartulum) t. elongatd, arcuatd, tereti, Icevi ; longitudin- 

 aliter interdum obsolete subangulaid ; albidd, seu hue ct illuc 

 semicorned ; aperlurd parum constrictd, dein minime reflexd, 

 parum declivi ; septo mucronato ; mucronem versus parum 

 ascendente marglne subrecto ; mucrone ad fronicm subelongatd, 

 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 resembles that of C. farci- 

 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 '115, '017027. 



Sab. Mazatlan ; extremely rare, off Spondylus ; ISpool Col 



Tablet 1550 contains 3 sp. of different ages, the middle one 

 possessing its operculum. 



Tablet 1551 contains 3 pseudo-csecous bodies, inserted for 

 comparison. The largest of these, measuring *045 across, was 



