156 . SHELLS AND SHELL-FISH. PART I. 



questionable ; but, until we know what the structure of 

 this animal really is, nothing further can be determined. 

 A similar notch is found in the genera lanthina, Scicu- 

 rella, and Pleurotomaria, among the phytophagous fami- 

 lies ; and the slit, in certain EmarginulcB, may probably 

 be analogous. By cUsposing the genera in the order they 

 have here been placed, some interesting analogies will 

 come to light. 



Analogies of theVh^unoTom^M and the Strombid.^ 



Genera of the ^ 7 • , /^t ^ Genera of the 



PiEUROToMiN^. Analogical Characters. Strombid*. 



Brachitoma. [ Outer lip gibbous and ascending ; 1 Strombidea. 



(_ inner generally thickened. J 



Pleurotoma. [Typical; canal considerably S pterocera. 



i lengthened, as long as the spire. 3 



Clavatula. Sub-typical ; canal short. Strombus. 



Clavicantha. Canal very slight. Aporrhais. 



ToMELLA. Inner lip considerably thickened. Rostellaria. 



(146.) We now turn to the Cerithin^, or club-shells, 

 the last division of the family. They merit this name, 

 not merely from their shape, but from having the whorls 

 encircled, either with obtuse spines, nodules, or granu- 

 lations. To M. Quoy we are indebted for a knowledge 

 of the animals of three of the genera, each of which has 

 some marked and peculiar character, — more so, even, 

 than their shells. Hitherto, with the exception of 

 Potomis and Pirena, they have been placed in one 

 genus. These two, from being covered with a strong 

 epidermis, are presumed to be fluviatile, or, at least, 

 living in the estuaries of rivers ; the rest are found in 

 marine but generally shallow depths, and often in pools 

 left by the tide. The species are numerous, both in a 

 recent and fossil state, particularly in the newest form- 

 ations, where some are stated to occur precisely simi- 

 lar to those now living. The whole of these shells have 

 a very long spire, a small body-whorl, and the outer 

 lip considerably dilated. The first genus is Brongniart's 

 Potomis, which may be called the coronated type, be- 

 ing encircled with obtuse spines and granulated points ; 



