420 MAZATLAN UNIVALVES 
plica declivi intus conspicuo extus ad labrum sinuatum circum- 
eunte ; operculo tenuissimo, vie spirali, striulis rugosis radian- 
tibus ornato. . 
As no fewer than 500 specimens were found of this typical 
species, (a rare number for a Pyramidellid,) most of them in 
very fresh condition, it is possible to describe it with tolerable 
accuracy. It is distinguished from its neighbours by the basal 
strie which are numerous, close and fine, beautifully adorned 
between with dots more or less elongated. The outer lip is 
produced and broadly indented at the base, narrow above and 
pinched at the suture, where it joins the labium which is 
often rather separate from the body whirl. The plait winds 
round the axis and develops into the boundary of the basal 
sinus. The sutural groove is not carried over the base, but is 
formed simply by the transverse ribs ending just before they 
reach the next whirl. These are not parallel to each other, 
but to the diverging outlines of the spire. They are very 
conspicuous on the young shell, which is broad and rather 
angulated at the periphery. As the adult shell narrows itself 
in, the base is produced and rounded, the ribs become some- 
what indented by the spiral lines, and near the mouth fade 
away altogether. In the adolescent state the labrum is often 
beautifully serrated by the spiral strie ; in the adult the lip is 
still transparent in the living shell. Its great thinness causes 
it to be frequently broken and repaired, the fresh part bemg 
not unfrequently almost devoid of sculpture. The spire is 
sometimes encrusted with coralline during the life of the animal. 
The operculum, (traces of which were observed in only about 
a dozen specimens,) is large, extremely thin, transparent, 
sometimes shewing a slightly spiral element, and covered over 
almost the whole surface with most minute wrinkles, radiating 
in curves from the centre like the engine-turning of a watch. 
Not only the size, but the number of whirls varies in different 
specimens known to be adult by the shape and evanescence of 
the sculpture. Very few are so large as those from Panama, 
and they generally make one turn less. The smallest specimen 
measures ‘03 by ‘021; the smallest adult, °053 by ‘025; an 
unusually large one, long. °12, long. spir.*085, lat. (in med.) 
"048, div. 23°. 
Hab.—Panama; 90 sp. under stones near low water mark ; 
C. B. Adams.—Mazatlan ; abundant and fresh among alge 
on Uvanilla, somewhat rare on Chama, Spondylus, &c. : 
L’pool & Havre Coll. 
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