394, MAZATLAN UNIVALVES 
name was pre-occupied by Sow. The inaccuracy of the figure 
in the Conch. Ic. and the variation in the number of spiral 
tubercular striz (in this species not a constant character,) excuse 
Menke for having redescribed it. The P. atrior of Prof. 
Adams, described from a single specimen, appears to be cliar- 
acterized by the pale spiral stripe; which, though rarely con- 
spicuous, and not noticed in the diagnosis of the species, may 
generally be seen inside the mouth, by holding the shell up to 
the light ; and in fine specimens is often clearly discernible on 
the last whirl. The P. discors scarcely differs in essential 
characters, though it is a much larger form. Shell with the 
entire surface very minutely spirally striated; with a prominent 
keel near but not close to the suture, which partially covers 
the principal row of nodules, 12—14. Below are several spiral 
 lirule of which generally two, sometimes more, are prominent 
and granular, the granules corresponding with the tubercles 
above. Sinus rather narrow and deep, in the middle of a large 
excavated area; with the thick posterior callosity labral rather 
than parietal. A narrow horn-coloured band, sub-transparent, 
runs just above the tubercles ; (in one specimen broad, cover- 
ing the tubercles,=P. atrior, C. B. Ad.) Operculum broader 
than in D. luctuosa, of a deep rich purplish red colour, very 
glossy inside. The spire outlines are very convex in the adult, 
which has somewhat the aspect of a small D. rudis. The small- 
est adult sp. (decollated) measures '34 by ‘15. The longest, 
long. *72, long. spir. °36, lat. ‘24, div. 35°. (anfr. sup.) 
The broadest sp. measures Jat. ‘28, div. 40°. 
Hab.— Monte Christi; under stones, Cuming.— Panama ; 
14, sp. do. at low water mark; C. B. Adams.—Mazatlan ; 
not common, often incrusted with coralline, very rarely with 
mud; L’pool Col. 
Tablet 1891 contains 8 specimens, different ages.—1892, 4 sp. 
spire more elevated.—1893, 3 sp. acuminated ; sculpture some- 
times nearly obsolete.—1894, 3 sp. dwarf state.—1895, 2 sp. 
light band developed.—1896, 2 sp. with opercula, one normal, 
the other with nucleus subcentral ; and one separate operculum. 
462. P DRILLIA CERITHOIDEA, 7. s. 
2D. t.* D. aterrime” similt ; sed carine infrasuturali carente, 
(interdum stria majore hic monstrante,) ared sinus angustiore, 
haud excavata, superiore ; monilibus spiralibus in bast pluribus ; 
tuberculis in spird magis conspicuis ; labro serrato, sinu pro- 
