CHAP. VII. MONODONTA. THE SUB-GENERA. 219 



several species, but little known *, are found in the Medi- 

 terranean ; and these are the most trochiform, that is, 

 the basal whorl is unusually flattened, which renders the 

 aperture narrow : those from the Pacific, on the contrary, 

 are more ventricose, thus representing Cidaris and the 

 Senectince. This modification of form is to constitute 

 the passage which here takes place between the five types 

 of Trochus and those of Monodonta. The last sub-genus, 

 Pagodella, is the only one containing shells that are not 

 perlaceous. The student, at first sight, would think this 

 was a most heterogenous group, for it contains species of 

 very different forms: some are so like European Callio- 

 stomfe, that they might be strictly arranged as such, if 

 the substance of their shell was not regarded ; others 

 are equally conic, but instead of being smooth, are beset 

 with nodulous granules ; while others, again, are formed 

 precisely the same as our first type, Chlorostoma, but yet 

 have no umbilicus. But this is at once explained by 

 the situation of Pagodella, which is intermediate between 

 Cattiostoma and Chlorostoma. So exquisitely, also, has 

 nature blended this genus with the last-named group, 

 that there is one species, the Trochus Merula of La- 

 marck, which unites in itself still more closely the cha- 

 racters of Chlorostoma and Pagodella ; it has the shape, 

 colour, and pearly substance of the first, and the flat- 

 tened imperforate lip of the last ; it may, in fact, be 

 called either a perlaceous Pagodella or an imperforate 

 Chlorostoma. Having now gone through the genus 

 Trochus, we proceed to the next, or sub-typical group. 

 (205.) On entering the genus MONODONTA, we must 

 refer to what has been just said on the Oceanic | spe- 

 cies of Calliostoma, the greater convexity of whose basal 

 whorl cannot fail to have been remarked; thus we are con- 

 ducted to Elenchus, the first sub-genus of the present 

 group, and found only in the same latitudes. These 



* It is of these, we believe, that the genus Margarita has been pro- 

 posed, to include such as have " the operculum of few whorls." 



t The Continental naturalists employ this term to designate the pro. 

 ductions of those countries or seas lying in the Great Pacific Ocean. 



