MAZATLAN UNIVALVES 327 



specimen found in the adolescent state, these ribs remain 

 between smooth portions, shewing the position of a former 

 mouth. A.n immature specimen displays, under a high power, 

 extremely minute longitudinal irregular wrinkled striae, which 

 are probably due to the epidermis. One operculum was found 

 in situ, which appears somewhat less concave than in C. Iseve, 

 but it was not extracted. An unusuajly large sp. measures 

 long. '064, lat. "019. 

 Hob. Mazatlan ; very rare, off Spondylus, *&c. ; 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. CAECUM GLABEIFOEME, n. S. 



C. (Fartulum) t. " C. glabro" simillimi, sed septo mamillato 

 tumentiore,ferc Jiemispherico ; 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. '017. 

 Hob. 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. * 



375. CAECUM COEEUaULATUM, n. S. 



C. (Fartulum) t. " C. glalriformi" simili, sed tumidiore ; 

 superficie tenuissime transversim striatd, striulis ndnimis irregu- 

 lariter corrugata ; septo ?mammillato ; operculo ? 



