MAZATLAN UNIVALVES 293 



Crucibulum spiuosum begins life as a smooth. VelutiDa- 

 shaped body, about '03 across, scarcely diflt^ring from the 

 yotuig of C. imbricatiim. Sow. As it spreads out, it commences 

 its cup as a Crepidvdoid deck, growing like that of Crepidula 

 nivea, but at a greater angle. This is soon developed into a 

 semicircidar lamina, much flattened at tlie posterior end, and 

 in the same proportion produced anteriorly. On the smooth 

 surface are first developed fine strife ; then rather irregular 

 but fine corrugations, radiating to the margin as in Patella, 

 not folloviing the spiral grovrth of the shell. These corruga- 

 tions are sometimes decussated by the lines of growth, which 

 occasionally develop concentric lamina?, as in Crei)idida 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 disai^pear, 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, whicli was spiny in its earlier stage, goes 

 on smooth. In shape there is the same diversity. Most of 

 the young sliells are ratlicr conical : but they generally deve- 

 lop in a more or loss expanded form, sometimes nearly flat. 

 Mr. Cuming has a most wonderful specimen (figured in the 

 Trans. Zool. Soc. pi. 28, f. 8), which he extracted from a spheri- 

 cal 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 tbis creature must have been perfectly sedentary. 

 Other specimens however, after beginning with an uneven aud 

 broken margin, appear to have gone in quest of more quiel 

 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. imbricatuia. 

 It is always flattened at the inner posterior portion, with a 

 more or less defined angular groove, separat^g the flat from 

 the swollen portion. It is white, with transparent linea of 



