222 SHELLS AND SHELL-FISH. PART I. 



from Solarium to Onustus on one side, can also be dis- 

 covered on the other side from Canthorbis. The gra- 

 dation, in fact, is equally perfect on both sides : the 

 two extremities of the column of the Trochin^, com- 

 mencing with the sub-umbilicated species of Canthorbis, 

 and ending with the sharp-edged Onustus, meet to- 

 gether in the Onustus agglutinans * , which thus closes 

 the circle of the five genera of the Trochin^. 



(207.) The two aberrant types of this family are as 

 remarkable for the paucity of their forms, as the typi- 

 cal Trochin^ are otherwise. Of the Rotellina;, in fact, 

 we can only enumerate two genera ; and one of these not 

 so effectually as we could wish. We think the Turbo Ni- 

 cobaricus is the type of a sub-genus {^Chi-ysostoma Sw.) 

 much more related by its shell to Rotella than to 

 Senectus, even if its operculum should prove to be 

 shelly. It diflPers from all other Trochidce, in having 

 a very thick deposition of shelly matter spreading over 

 the umbilicus, which it almost conceals : it does not, 

 however, extend near so far as in Rotella; and yet both 

 shells are evidently highly polished by their animals. 

 On this account, therefore, and in the absence of all 

 knowledge of the animal, we place it as the represent- 

 ative, among the Rotellince, of Senectus and Monodonta. 

 Of the fossil genus Pleurotomaria very little can be 

 said : it obviously enters into our present family, from 

 the trochiform shape of its shell ; and the foregoing 

 analysis renders its station in any other group highly 

 improbable ; we place it, therefore, between Rotella 

 and Phasianella, as the type of a sub-family, the other 

 members of which cannot now be distinctly ascertained. 

 The truth is, that in this and numerous other instances, 

 the study of fossil conchology is impeded by insur- 

 mountable difficulties, which must always exist. How 

 many genera are in our systems, belonging to the older 



* It is somewhat remarkable, that, from Lamarck's description of this 

 shell, it would seem to have the umbilicus open when young, but closed 

 when it has reached maturity. In two fine specimens now on the table, it 

 is completely covered ; and yet there is a fossil species from Hordwell, 

 where it is perfectly open, although not large. 



