294 TROCHHXE. 



fossil. It is evidently of great antiquity, although 

 palaeontologists are not agreed as to its origin. Sowerby 

 assigns this probably to the Lias, Woodward to the 

 Devonian formation, and Searles Wood to the Protozoic 

 rocks. The distribution of existing species corresponds 

 in extent with their number ; none of the typical form 

 appear to inhabit North-east America only those of 

 the section Margarita. 



For the same reasons which I gave in the preceding 

 volume for not dismembering Venus as regards the 

 British species, I will preserve Trochus in its integrity, 

 at the same time dividing it into as many sections as 

 the gradual nature of the differences between the species 

 may seem to warrant. It is true that all the species 

 comprised in the so-called genus Margarita are quite 

 pearly and that some of them are low-spired and umbi- 

 licate ; but it must be observed that Trochus occiden- 

 talis (which is placed by Loven in that genus) , although 

 pearly,, is high-spired and has no umbilicus, and that 

 T. Vahlii and T. amabilis are decidedly conical. The 

 shells of Gibbula are usually low-spired and deeply um- 

 bilicate; but varieties of T. tumidus, T. umbilicatus, 

 and T. cinerarius (referred to this genus by the Messrs. 

 Adams) have the spire raised, and the base is not even 

 perforated. Searles Wood says that in Crag specimens 

 of T. tumidus (which connects Gibbula with Margarita) 

 the umbilicus is very variable ; " in some it is open, 

 while in others it is quite covered, depending upon the 

 elevation or depression of the spire, and also on the 

 extension of the left lip." Again, T. lineatus is our only 

 representative of Klein's genus Trochocochlea, in which 

 the spire is raised, the base imperforate, and the pillar 

 lip furnished with a blunt tubercle or notch ; the last 

 two characters are common, however, to several species 



