The following is a list of the genera, -to which all the species of Twhi- 

 uacea at present known may be referred : — 



Genus 1. TURRITELLA, Lamarck. 



Animal ; disk short, oval, surmounted hy a stout pedicle which 

 serves as a support for the head and enters the shell ; head 

 prolonged into a cyli7idrical rather jlattened trunk, broad at the 

 base arid cleft at its anterior extremity ; tentacles elongated, 

 with the eyes at the outer base ; mantle forming a fringed ring 

 or collar, variously ornamented according to the species, through 

 which the head passes in and out of the shell. Operculum 

 horny, multispiral. 



ISliell; very long, narrow, turreted ; whorls numerous, generally 

 transversely, never longitudinally, ribbed, devoid of spines or 

 tubercles, convoluted into a spiral screw ; aperture small, some- 

 what round, loith the margins disjoined, lip sharp, never re- 

 flected, broadly sinuated towards the upper part. 



In speaking of the genus Terehra (ante p. 55), I observed that "the 

 shell of Turritella has very much the form of Terehra, but the aperture is 

 round and entire ; so that Terehra may be likened to a very long drawn-out 

 Bucc'mum, and Turritella to a similarly elongated Turbo." The comparison 

 cannot, however, be contiimed with the same force, for the shell of Turbo is 

 of a soUd pearly composition, whilst that of Turritella is not, and the animals 

 are somewhat dissimilar, though the shell of both is alike distinguished by 

 its rounded apertm-e and by the absence of any basal canal or sinus. It 

 is, indeed, a matter of doubt whether so much importance can be attached, 

 as hitherto, to the canaliculated structui-e of shells. According to the 

 observations of M. Deshayes and MM. Quoy and Gaimard, there must be 

 a closer relationship than has been yet acknowledged between the Titrri- 

 tetlm and the Ceritkia and Melauice, although the shell of one genus is 



