228 MAZATLAN UNIVALVES 
wing, so that the expansion of each penultimate whirl lies 
closely over the next. A depressed portion, bounded by two 
spiral lines, appears between the body and the wing, forming 
a false suture on the spire. Base with very fine strie of 
srowth ; with a nodulous spiral raised keel about the middle, 
and a nodulous line within, bounding the umbilical region. 
This is of a lustrous orange red, shading into black over a dull, 
sometimes slightly bifid spiral subcentral keel. The umbilical 
mark is but moderately excavated, with a slight central cal- 
losity. Labrum extending one-fourth of a whirl. Parietal 
labium covering the inter-carinal region for another fourth. 
The rest of the shell (base and spire) is covered with an 
oblivaceous epidermis, lying in strongly striated lines of growth. 
Colour beneath the epidermis dark green, more or less tinged 
with red, especially in the young shell. The medial portion, 
wing and tubercles, are often very slightly shewn in the adult. 
The shell when in good condition is extremelv beautiful ; but 
it is almost always covered with animal and vegetable incrus- 
tations. It is subject also to the attacks of Lithophagi and 
Gastrochene, which generally have the instinct to burrow 
along the thick winged portion, or down the axis; but often 
venture to invade the liver-chamber in the upper whirls, when 
the Trochid withdraws itself, forming partitions as in Turritella, 
&e. Operculum smooth, outside with two strong ridges, begin- 
ning from nearly the same point, and meeting at the other 
end; of which the outer is far larger; with a deeply-scooped 
hollow within ; and a faint ridge at the outer margin. Inside 
obtusely raised at the back of the large ridge ; with blackish 
brown horny covering. The largest sp. measures Jong. (from 
apex to wing at base) 2°9, lat. ‘26, div. 65°. A spreading sp. 
long. 1°9, lat. 2°15, div. 80°. 
Hab.—Mazatlan; extremely abundant; Z’pool Col.—S. W. 
IMiexico,.P2P: GC: 
Tablet 1088 contains 5 sp. different ages, spreading var.— 
1089, 5 do. usual form.—1090, 3 do. very conical.—1091, 4 sp. 
after going through the acid process, with the beautiful basal 
epidermis removed, but displaying the exterior markings.— 
1092, 4.sp. with base beautifully preserved.—1093, 2 sp. mended 
after fracture.—1094, 1 sp. with the mouth curiously distorted 
by an Ostrea conchaphila. Each party has raised its lip to 
escape the other.—1095, 1 sp. with Lithophagus aristatus in 
situ. Having bored through an empty barnacle as far as the 
interior, it has increased outwards, making an external tube. 
