68 MOLLUSCA FROM THE CRAG. 



spiral ridges, nearly equidistant ; and with fine longitudinal striae or lines of growth ; 

 aperture subcanaliculated ; left lip slightly reflected and flattened ; umbilicus open 

 and deep. 



Jxis, \ an inch. 



Locality. Cor. Crag, Sutton. 



Mam. Crag, Bridlington. Recent, Oban. 



I have as yet found but two or three specimens of this species in the Coralline Crag, 

 and these strongly resemble a recent shell in my possession obtained from the Bay of 

 Oban. In the fossil the spaces between the ridges are quite flat, with fine visible 

 lines of growth ; the recent shell has sometimes a small intermediate ridge, but these 

 difi'erences are not constant. Specimens from Bridhngton, from the cabinets of Messrs. 

 Bean and Leckenby, are of a less elongated form, and are thicker and stronger, with a 

 very flattened and purpura-like form of inner lip, and having a more open umbilicus, 

 with less prominent transverse ridges ; I believe it, however, to be only a variety. 

 Fig. 11, Tab. XIX, represents a specimen belonging to Mr. Leckenby which differs 

 from the recent form considerably more than the shell from the older formation. 



Cerithium,* Adanson. 1757. 

 BiTTiUM. Leach. MS. 1819. 

 PoTAMlDES. Brongn. 

 Trifobis. Besh. 1824. 

 PoTAMiDUM. Flem. 1828. 



Ge7i. Char. Shell elongate, or turriculate, with an elevated or pyramidal spire, 

 composed of numerous volutions ; ribbed, striated, or tuberculated, occasionally smooth ; 

 aperture subquadrate, terminating in a short recurved canal ; outer lip more or less 

 sinuated, sometimes expanded ; operculum corneous. 



In this genus Lamarck included a large number of species, both marine and fluviatile, 

 the latter of which have been separated and placed in a distinct genus by Brongniart 

 under the name of Potamides. There is no difference whatever in the shells them- 

 selves ; a species from the purely fresh-water formations of Hordwell has a deep and 

 recurved canal, which is the character generally considered as peculiar to the marine 

 species. Many of these species are inhabitants of estuaries, and capable of enduring 

 an existence either in fresh or salt water. The same species in the Mammaliferous 

 Crag is found with land and fi-esh-water shells, which in the Red Crag is associated 

 with purely marine forms. 



The greater number of species constituting this genus have a distinct and well- 

 marked canal at the base of the aperture, which, however, in some species, dwindles 

 into a slight inflection of the lip at the lower part of the columella, thus approaching 

 in appearance some of the shells in the genus Turritella. 



* The derivation of this genus is not well determined. Hermansen considers it probably from mjui's, 

 a kind of fish, or from uripvdov, buccinulum. Agassiz derives it from kepdnuv, a little horn. 



