138 MOLLUSCA FROM THE CRAG. 



A very abundant shell. It differs from the preceding, A. striatus, in being quite 

 free from stride on the upper part, and in having three keels at unequal distances upon 

 the volution, of which the lower one is the most prominent and permanent, and the 

 upper one, even the upper two, are sometimes evanescent. A small incipient sinus is 

 visible at the upper part of the aperture, which gives a depression round the volution 

 near, but not at, the suture, which character I do not observe in the other species. 

 This may possibly be the Skenea divisa, Flem. (Brit. An. p. 314), Turbo divisa, 

 Adams ; but these authors make no mention of the keel-like ridges that surround the 

 shell. 



Three or four coarse striae or ridges cover the volution at the umbilicus, and the 

 lines of growth are elevated at that part, and are corrugated. The most abundant 

 variety is that which is entirely smooth above, with only one keel upon the base or 

 outer edge of the volution. 



3. ADEORBIS TRICARINATUS. S. Wood. Tab. XV, fig. 6. 



ADEOKBIS TRICARINATUS. S. Wood. Catalogue 1842. 



A. Testa minutd, depressd, subdiscoided ; anfractibus subcylindraceis ; cingulis tribus 

 acutis, subequidistantibus, instructis ; interstitiis spiraliter striatis ; superne et in/erne 

 planiusculis, umbilico magno. 



Shell small, depressed, subdiscoidal ; with three convex or rather subcylindrical 

 volutions, furnished with three sharp subequidistant and elevated carinse spirally striated 

 between them ; shell rather flat above and beneath, with a large open umbilicus, slightly 

 corrugated by lines of growth. 



Diameter, \ of an inch ; altitude, \ its diameter. 



Locality. Cor. Crag, Sutton. 



I have about half a dozen specimens in my cabinet with the above characters, 

 in which they appear to diifer from the two preceding species sufficiently to be 

 considered specifically distinct. 



A. supra-nitida possesses the keels, but less prominently, and less equally distributed, 

 and it is perfectly smooth except in the umbilicus. A. striatus possesses the striae, but 

 has no vestige of carinse ; and this shell has not the coarse spiral striae of the umbilicus. 

 It is possible that they may be only varied forms of one species, but the numerous 

 specimens in my cabinet appear to preserve the above distinctions, and they must for 

 the present, therefore, remain separate. 



