MAZATLAN UNIVALVES 317 



363. CJECUM OBTUSUM, n. s. 



C. (ElepJiantulum) t. elongatd, tenue liratd, aperturam ver- 

 sus hand tumente, ; septo submammillato, parum tumente, apice 

 obtusissimo dorsum versus spectante ; operculo ? . . . . 



Six specimens and fragments were found of this species, 

 which in the liration 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 

 measures long. '04, lat. '01 '014. 



The largest '132, '017 '026. 



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



Tablet 1517 contains the largest and the smallest specimens. 



; 364. dECUM LIRATOCINCTUM, n. S. 



C. (ElepJiantulum) t. tenue liratd, liris plus minusve acutis, 

 confertis seu subdistantibus ; plerumque hue et illuc, maxime 

 aperturam versus, annulis incrementi obsoletis cinctd, sub liris 

 monstrantibus ; albidd, seu alba et corned concentrice undato- 

 nebulosd ; septo submucronato, subungulato ; mucrone dacty- 

 liformi ; apice plerumqwe maxime prominente, laterally acuto ; 

 margine laterali subconcaviter ascendente : operculo valde con- 

 cavo, anfr. circiter xii. lira 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 beautifully mottled 

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

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

 sometimes appear double. The plu^ 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 entral. 

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

 rather sinistral ; but as the ribs 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 



