MAZATLAN UNIVALVES 317 
363. CHCUM OBTUSUM, 7%. s. 
C. (Elephantulum) t. elongatd, tenue liratd, aperturam ver- 
sus haud 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 — O14. 
The largest 93 8255 SONI 02 
Tab.—Mazatlan ; extremely rare, off Spondylus ; L’pool Col. 
Tablet 1517 contains the largest and the smallest specimens. 
364. CHCUM LIRATOCINCTUM, 2. S. 
C. (Elephantulum) t. tenue lirata, liris. plus minusve acutts, 
confertis seu subdistantibus ; plerumque hue et illuc, maxime 
aperturam versus, annulis incrementi obsoletis cinetd, sub lirts 
monustrantibus ; albidd, sew alba et corned coneentricé undato- 
nebulosa ; septo submucronato, subungulato ; mucrone dacty- 
liformi ; apice plerumque maxime prominente, laterali, acuto ; 
margine lateral subconcaviter ascendenie: operculo valde con- 
cavo, anfr. circiter xii. lird prominente spirali, marginem tnter- 
dum quasi dwplicante. 
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 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 eentral. 
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 
