MAZATLAN UNIVALVES 293 
Crucibulum spinosum begins life as a smooth Velutina- 
shaped body, about °03 across, scarcely differing from the 
young of C. imbricatum, Sow. As it spreads out, it commences 
its cup as a Crepiduloid deck, growing like that of Crepidula 
nivea, but atagreater angle. This is soon developed into a 
semicircular lamina, much flattened at the posterior end, and 
in the same proportion produced anteriorly. On the smooth 
surface are first developed fine strie; then rather irregular 
but fine corrugations, radiating to the margin as in Patella, 
not following the spiral growth of the shell. These corruga- 
tions are sometimes decussated by the lines of growth, which 
occasionally develop concentric laminz, as in Crepidula Les- 
sonii. Over these corrugations appear, not in the same 
direction, but in lines more or less spirally diagonal, and more 
or less irregular, either small tubercles, or very small tubular 
spines, generally open at the top, and, when close to the 
margin, at the bottom also. The spines are often very long, 
either on a portion or on the whole of the shell. Some- 
times a few rows are predominant, as in C. hystrix, Brod. : 
sometimes all are alike and very crowded. In this state, 
the corrugations often disappear, developing themselves into 
spinous processes. It is common to find a shell smooth on the 
principal part, then suddenly developing spines at one corner ; 
or sometimes a shell, which was spiny in its earlier stage, goes 
on smooth. In shape there is the same diversity. Most of 
the young shells are rather conical: but they generally deve- 
lop in a more or less expanded form, sometimes nearly flat. 
Mr. Cuming has a most wonderful specimen (figured in the 
Trans. Zool. Soc. pl. 28, f. 8), which he extracted from a spheri- 
eal hole in a large stone brought up from deep water. After 
beginning in the usual conical way, it had spread out almost 
flat, the cup projecting far beyond the base of the shell, and 
the enormously large spines being recurved above the vertex. 
The life of this creature must have been perfectly sedentary. 
Other specimens however, after beginning with an uneven and 
broken margin, appear to have gone in quest of more quiet 
abodes, and have continued as flat and regular shells. The 
margin is either entire, slightly crenulated by the corrugations, 
or beautifully indented and pierced by the tubes, according to 
circumstances. The cup is adherent for about two-thirds of 
its height; very rarely to near the top, as in C. imbricatum. 
It is always flattened at the inner posterior portion, with a 
more or less defined angular groove, separating the flat from 
the swollen portion. It is white, with transparent lines of 
- 
