218 



SHELLS AND SHELL-FISH. PART I. 



easy, for it might then be formed to contain all those 

 small or moderately sized trochuses, which are without 

 a distinct tooth at the end of the pillar, and have a horny 

 operculum. But this definition would break up the 

 natural series. The group, in fact, is so difficult to cha- 

 racterise in general terms, that we suspect its funda- 

 mental difference from that of Monodonta lies in certain 

 peculiarities of the animals, with which at present we are 

 unacquainted ; but if, to the foregoing characters of 

 Trochus, we add that one of its sub-genera has a promi- 

 nent tooth, but no umbilicus, and that another is not per- 

 laceous, we shall then name the only exceptions. The 

 whole genus, then, divides itself into the following five 

 sub-genera: — 1. Chlorostoma, where the basal volution is 

 either perfectly flat or concave, the margin carinated, and 

 the outer lip so oblique that it extends half way round 

 the circumference of the shell ; the umbilicus is very 

 deep, and is bordered on one side by the inner lip, 

 which is suddenly truncate as soon as it reaches half 

 way round the umbilicus : this leads to Troclius, or the 

 typical sub-genus. All our British species, excepting 

 T. zizyphinus, come into this division ; the umbilicus 

 is more or less deep, and the aperture either entire or 

 with a very slight angle at the piUar : by degrees, 

 however, this angle is so much developed, that it assumes 

 the appearance of a tooth : thus we are led into the sub- 

 genus Trochidon, nearly all of which are natives of the 

 southern hemisphere. Some have the tooth as distinct 

 as in Monodonta ; but they may at once be known by 

 having no umbilicus, even although the inner lip may 

 have a slight marginal groove. But here, as nature has 

 reached the highest point of developement, she again re- 

 cedes ; the spire gradually lengthens, the tooth becomes 

 a simple angle, and we enter on the smooth division of 

 the group, forming our sub-genus CaUiofitoma. The 

 Trochus zizyphinus of British writers will give a very 

 good idea of these shells ,• they are nearly all either per- 

 fectly smooth or slightly granulated, of a light and ele- 

 gant form, a long and pointed spire, and no umbilicus : 



