CHAP. VII. TROCHID.E, SENECTIN^. 215 



sphere. The ventricose form of the body-whorl of, 

 course, modifies the shape of the aperture, which is thus 

 always circular, and but seldom oblique. Before we 

 had sufficiently studied this family, we included the 

 foregoing in our genus Marmarostoma ; but we intend 

 to limit that name to the umbilicated division of Hum- 

 phrey's Senectus, represented by the M. versicolor*, 

 the passage from one to the other group being made by 

 our Senectus coronatus.^ The umbilicus in these is, 

 indeed, small, but very deep ; the spire is almost perfectly 

 flattened, the tip obtuse, and the base even more pro- 

 duced than in Senectus. It is quite clear to us, that 

 more than one species is confounded by conchologists 

 under the specific name of coronatus; since some have 

 an umbilicus, and others not. In all the Marmaro- 

 stonifB, however, the pillar is present ; but on entering 

 upon Lamarck's Delphinula, the umbilicus is open to 

 the terminal spiral whorl, and there is no pillar : the 

 Turbo torquatus of the old conchologists is, therefore, 

 a true Delphinula, connecting this genus with the last. 

 Of the fossil shells referred to Delphinula we shall not 

 speak ; judging from their figures, and from a few speci- 

 mens we possess, they appear to require a thorough 

 revision, and to contain types very different from those 

 which are recent. Our next genus, if it be really one, 

 contains, at present, but two species, differing in being 

 very slightly perlaceous : they may be compared to 

 Delphinula without an umbilicus. The name of Cyclo- 

 cantha may explain their round form, and the circle of 

 spines on the body-whorl. Cidaris is the last genus, 

 and contains those Senecti which have the base desti- 

 tute of any lobe, the aperture more oblique, the apex 

 of the spire obtuse, and the outer surface almost always 

 smooth J; the aperture is quite circular, and closed by 

 a thick calcareous operculum. There are many species, 

 of which the beautiful Cidaris sarmaticus may be 



* Turbo versicolor Martini, pi. 176 fie. 1740, 1741. 

 t Ency. Meth. 448. fig. 2. 



J Except in our S. coronata, which connects this sub-family with the 

 next, 



P 4 



