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. 



Axis, ^ 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 differences are not constant. Specimens from Bridlington, 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. 



Ceeithium,* Adanson. 1757. 

 Bittium. Leach. MS. 1819. 

 Potamides. Brongn. 

 Trifobis. Desh. 1824. 

 Potamidum. Flem. 1828. 



Gen. 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 themselves ; 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 fresh-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 <o?pi'*, 

 a kind of fish, or from KripvKiov, buccinulum. Agassiz derives it from Kipanov, a little horn. 



