RISSOA. 149 



L. crassior also has a produced spire, but is rather anguL^r 

 and tliick, and has scarcely any umbilical groove. 



III. AssiMiNEA Grayana is a small mollusc, with a dull, 

 horny shell ; found in great abundance on the marshy bor- 

 ders of rivers where the water is brackish or fresh. It was 

 first noticed by Mr. J. E. Gray at the Greenwich marshes, 

 and described in the fifth volume of tlie ' Zoological Journal 

 by the Eev. M. J. Berkeley. The muzzle of the animal is 

 lobed in front ; there is a groove running backwards on each 

 side from the base of the tentacula, which are short, and 

 have the eyes placed on their tips ; the shell is Troclms- 

 shaped, and about the sixth of an inch long. 



lY. EissoA is a genus of rather minute, some of them 

 microscopic molluscs, which have very long tentacles, with 

 eyes at their thickened bases ; the foot is pointed at 

 the back, and bears a horny operculum ; it is produced in 

 front of the muzzle. The shells are generally rather pyra- 

 midal, and greatly varying in sculpture in difl*erent species, 

 which are so numerous and minute that we shall only men- 

 tion a few of the most remarkable characters : — 

 R. labiosa : a produced tapering spire, with elegantly turned 

 longitudinal ridges; smooth, with an expanded and 

 thickened outer lip. 



