13 4 MOLLUSCA FROM THE CRAG. 



half of the volution, and the same number are on the base, with only three between 

 the keels ; those upon the base are most prominent and distinct. 



In Savigny's great work upon Egypt is the representation of a shell somewhat 

 resembling this species, but which, from the figure, appears to be too strongly imbri- 

 cated, and to have the aperture too circular, to be considered identical ; a comparison 

 with the shell itself is necessary for such determination. 



Many of the shells in this genus also have the exterior much altered by decorti- 

 cation, and the determination of some of the species is not by any means satisfactory. 



MARGARITA,* Leach, MS. 1819. 

 TKOCHUS (spec.) Linn. 

 SOLARIELLA, S. Wood. 1842. 



Gen. Char. Shell thin, subconical, trochiform ; spire moderately elevated ; volutions 

 few, rather inflated ; striated, sometimes costated ; aperture subcircular ; peritreme 

 sharp, discontinuous, interrupted by the previous volution ; umbilicus generally large 

 and deep ; operculum corneous, multispiral. 



The animal of this genus much resembles that of Trochus, and it is considered by 

 some conchologists as not deserving of generic distinction. There is, however, a 

 delicacy and generally a semitransparency in these shells, accompanied by a large and 

 open umbilicus in all the species, that at least entitles them to a sectional separation, 

 and as the name was employed in my Catalogue, I have retained it here. 



Mr. G. B. Sowerby, sen., has described fifteen recent species of this genus in 

 the ' Malacological Magazine,' 1 838, all of which are northern forms. 



1. MARGARITA ELEGANTISSIMA. Bean, MS. Tab. XV, fig. 1. 



M. Testa orbiculari, conoided, tenui ; apice acuto ; anfractibus quinque, superne con- 

 vexis y in/erne planulatis ; longitudinaliter oblique costulatis ; interstitiis tenuissime striatis ; 

 umbilico magno, bicarinato. 



Shell orbiculato-conoidal, thin, and fragile, with a slightly elevated spire and acute 

 apex ; volutions five, convex above, rather flat beneath, outer edge obtusely angular ; 

 longitudinally costated, with elevated, reflected, or imbricated costae, finely striated 

 transversely ; umbilicus large, bicarinated. 



Axis, \ of an inch. 



Locality. Mam. Crag, Bridlington. 



The cabinet of Mr. Bean, of Scarborough, has furnished me with the only specimen 

 of this species that I have seen. It appears to be distinct from any with which I am 

 acquainted, and I have, in accordance with that gentleman's opinion, considered it 

 a new species. The characters which most distinguish it are the prominence of the 



* Etym. Margarita, a pearl ; from the nacreous character of these shells. 



