MAZATLAN UNIVALVES 388 
which however it is distinguished by the absence of the lower 
row of tubercles. There are about 16 whirls, of which the 
first three are smooth, brown and translucent ; while the next 
three (about) display the coste without the spiral groove. The 
young shell is very dark coloured, with the ribs conspicuous ; 
the girdle gradually develops its white colour, and afterwards 
very irregular reddish brown spots. The coste gradually 
become fainter and more irregular, and are or are not decussated 
by the spiral lines. These are normally 4--6, but variable. 
The whole surface is covered with microscopic spiral striule, 
irregularly waved, giving arugose appearance. The sutural 
line is continued round the base in a faint white band. The 
deeply eut notch is bounded by a sharp keel externally, which 
runs round over the columella. The colour is almost always 
purplish brown, shading into olivaceous brown, more or less 
dark, lustrous. Operculum rhomboidal, more angular than in 
Pleurotoma, apex at the end of the long diagonal, expanding 
with straight sides and a scarcely rounded end. Most of the 
opercula found are abnormal, mended as from a subcentral 
nucleus. These beautiful shells are often encrusted with black 
mud, as in Drillia luctuosa, and have frequently been obliged 
to repair breakages at their extremity. The largest sp., unusu- 
ally slender, measures Jong. 1°6, long. spir. 1°2, lat. °34, div. 13°. 
A young broad sp. pear 4 0,03 i FAs! 28M s Oe 
Hab.—Mazatlan ; not common; L’pool Col. 
Tablet 1828 contains 5 sp. usual colour.—1829, 5 sp. richly 
olivaceous.—1830, 3 sp. purplish brown predominating.—1831, 
1 sp. ashy tint, markings faint.—1832, 2 sp. markings very 
faint.—1833, 3 do. markings very strong.—1834, 38 sp. repaired 
after simple and compound fractures.—1835, 1 sp. with opercu- 
lum in situ; and 2 separate opercula, one normal, the other 
mended after fracture. 
451, Myvuretta Hinpstt, ?n. s. 
M. t. “M. albocinete” simili, sed graciliori ; anfr. confer- 
troribus ; omnino albidd, rufo-fuseco vix tinctd ; cingulo aream 
majorem occupante, tuberculis validis ; costis evanidis ; lineis 
sptralibus i.-v., striulis haud apparentibus ; plicd columellari 
haud conspicud. 
A very few specimens were found of a whitish colour, faintly 
spotted with reddish brown; with the ribs scarcely apparent, 
and the general surface smooth. Whether the form be of 
Nov. 1856. kk 
