MAZATLAN UNIVALVES 317 



363. CiECUM OBTUSUM", n. S. 



C. (Elephantulum) t. elongata, terme lirata, aperturam ver- 

 sus haud tumente, ; septo submammillato, paricni tumente, apice 

 obtusissimo dorsum versus spectante ; operculo ? . . . . 



Six specimens and fragments were foimd of this species, 

 whicli in the lii"ation closely resembles C liratocinctum, but 

 appears quite destitute of rings. It is known however by the 

 plug, which is almost mammillated, but has a very obtuse apex 

 pointing towards the back. The smallest of the specimens 

 measui'es long. '04, lat. '01 — "014. 



The largest „ -132, „ '017— -026. 



ITab. — Mazatlan ; extremely rare, off Spondylus ; L'pool Col. 



Tablet 1517 contains the largest and the smallest specimens. 



364. C.ECX7M; libatocinctum, n. s. 



C. (Elephantulum) f. teuue lirata, liris plus minusve acutis, 

 confertis seu subdlstantibus ; plerumque hue et illuc, maxime 

 aperturam versus, annuUs incrementi obsoletis ciiicla, sub liris 

 ■monstrantibus ; albidd, seu alba et corned concentrice undato- 

 nebulosd ; septo submucronato, subungulato ; mucrone dacty- 

 liformi ; apice plerumque maxime prominente, laterali, acuto ; 

 margine laterali subconcaviter ascendente : operculo valde con- 

 cavo, anfr. circiter xii. lird prominente spirali, marginem inter- 

 dum quasi duplicante. 



This very variable species shews a tendency both in its young 

 and old stages, sometimes very much, sometimes scarcely 

 developed, to form rings of growth under the ribs, which cross 

 them like the varices of Scalaria. The ribs are sometimes fine 

 and close, sometimes strong and rather distant, sometimes 

 degenerating into mere angles. A few very fresh specimens 

 were found, with white and horny brown beaviti fully mottled 

 in concentric portions. The whirls of the opercidum are fur- 

 nished outside with a very strong rib, which makes the margin 

 sometimes appear double. The plug is generally extremely 

 prominent at the side, flattened anteriorly, and rising into a 

 finger-like pointed knob behind. Sometimes however the 

 whole surface rises, and the apex becomes rather more tentral. 

 llarely in very young shells (when it is extremely sharp) it is 

 rather sinistral ; but as the ril)s then take a somewhat spiral 

 line down the tube, it is presumed to be merely accidental. 

 Only imperfect specimens of this form have yet been seen from 



