310 MAZATLAN UNIVALVES 



slowly enlarging sliell ; and by the plug, which is nearly flat, 

 with a very small knob on the right side of the back. This, 

 under a high power, appears to be of a subspiral growth, like 

 a tiny VitrineUa fixed on the surface. The operculum appears 

 to be thinner than that of C. liratocinctum, with rather more 

 numerous whirls and a fiiier spiral ridge. It is known at once 

 from the dextral specimens of that species by the very fine 

 ribs being straight, never spiral, and by the apex being round, 

 not pointed. 



The youngest specimen measures long. '05, lat. "007 — '01. 



The largest „ „ „ 'LdS, „ -Oio— 'OS. 



Sab. — Mazatlan ; ofif Spondylus ; extremely rare ; I! pool Col. 



Tablet 1515 contains 2 very young specimens, the largest 

 adult, and a fragment retaining its operculum. 



362. C^CUM ABNOEMALE, n. S. 



C. (ElepJianiulum) t. (lahnormaliter) curtissima, arcuata, 

 latiore, tenuiore ; Urtdis tenuissimis circiter xxx. ornata, aper- 

 turam vnricosam tumentem supereuntibus ; interstitiis inferdum 

 minutissime eoncentrice striatis ; septo mucronato, prominente ; 

 mucrone omnino dextrorsum lateraliter sito, ohtusiore ; margine 

 laterali (i. e. hoc specie, dorsali) concavo, rapide ascendente ; 

 operculo ? . . , . 



Whether this be an extremely short deformed variety of 

 C. subspirale, or a very aberrant species, cannot be affirmed 

 imtil more specimens are found. It is common among Caca, 

 in the process of growth, for specimens of the same age to vary 

 greatly in length ; but these shells, as shewn by the stout 

 varix at the mouth, are mature ; and yet, contrary to the habit 

 of Elephantulura, they are much shorter in proportion even 

 than in the compact group Inflatulura. They ai-e so much 

 bent that notwithstanding their shortness, both the surface of 

 the plug and the interior of the mouth can be inspected with- 

 out altering the position of the shell. Another very singular 

 feature is the position of the plug, which is set on the extreme 

 right, so that the front (or back) view of the shell gives it« 

 profile. One specimen measures long. "06, lat. "023; the other, 

 long. -048, lat. -018. 



Hab. — Mazatlan ; 2 sp. olF Spondylus ; L'pool Col. 



Tablet 151G contains one specimen. 



