522 MAZATLAN UNIVALVES 
with slightly developed but very numerous varices. The shells 
doubtfully assigned to Mr. Reeve’s ambiguous species by Prof. 
Adams, appear to be finely grown specimens of M, radix.* 
Shell of light growth, with a variable number of varices, 
generally 7 in the young and 9 in the adult state, with a vari- 
able pattern of open, more or less pinnate fronds, black within. 
These fronds are the expansions of rounded spiral ribs. The 
whole surface often very finely spirally striated. Base of 
columella twisted: canal nearly covered, bent, leaving a rather 
large spiral umbilicus. Labium generally stained with black. 
Groundwork of the shell white, with black ribs. Operculum 
ovate, much bent, nucleus near the margin, with irregularly 
laminated ridges of growth; on the outer border rising into 
very crowded erect serrated lamine ; colour dark horny brown, 
scarcely tinged with orange or red ; muscular scar large, 
corrugated like the human hand from a center on the inner 
margin. The smallest of the specimens, (including spines,) 
measures long. 1°75, long. spir. °52, lat. 1°25, div. 80°. 
ihelarcest: 53° 7; eee set age te eo Oy, 
Hab.—Mazatlan; abundant and extremely fine, among very 
fine sand ; L’pool & Havre Coll. 
Young state——Tablet 2481 contains 5 young sp. passing from 
form nigritus to form ambiguus.—2482, 1 sp. jun. six varices.— 
2483, 1 do. six and a half.— 2484, 1 do. seven.—2485, 1 do. seven 
and a half.—2486, 1 do. eight.—2487, 1 do. eight and a half.— 
2488, 1 do. nine. : 
Form nigritus, Phil.—2489, 1 sp. seven varices.— 2490, 1 do. 
very fine, seven and a half.—2491, 1 do. eight.—2492, 1 do. 
from a shop, locality unknown, displaying nine varices. 
Form ambiguus, Rve.—2493, 1 sp. dark, smooth, (lip rather 
broken) eight varices. The middle part of the last three 
varices takes the form nigritus, in consequence of an accident.— 
2494, 1 sp. nine varices.—2495, 1 sp. labium scarcely stained, 
spire outlines excurved, ten varices.—2496, 1 sp. (lip rather 
broken) labrum dark, eating away the lower part of an old 
varix, eleven varices.—2497, the largest sp., twelve varices.— 
2498, 1 sp. thirteen varices. 
Almost all the aboye specimens have opercula. Tablet 2499 
contains 3 others, of which two are distorted, with subcentral 
nucleus. 
* Of this species, though now extremely common, Lamarck thus writes : 
‘Shell very rare and yery precious. Ido not possess it; but I have had an 
opportunity of observing and examining its characters.” 
