184 Binney on Terrestrial Mollusks. 



This order is made well known by the genera Ci/cios- 

 toma and Helicina. It contains many other foreign gen- 

 era, but these two and Truncatella are the only ones in- 

 habiting the United States. 



It is divided by Pfeiffer into two suborders : Opisoph- 

 ihalma and EctopldliaJma^ respectively characterized by the 

 eyes being placed behind the base of the tentacles, and at 

 their external base. The 



SUBORDER OPISOPHTHALMA 



is characterized as above, and contains one family only, the 



FAMILY ACICULACEA. 



In addition to the characters of the order, it has a thin 

 spiral operculum and few whorls. It contains the Euro- 

 pean genus AcicKhi, and the genus Geomelanin, of Jamaica, 

 as well as Truncatella, which last alone inhabits the United 

 States. 



Gknus truncatella Risso. 



Shell imperforate, but with an umbilical groove, cylin- 

 drical, turretted, usually pellucid and smooth, of a reddish 

 horn-color; the upper whorls are also truncated in the 

 adult, the remaining ones are usually gradually increasing 

 in size, and covered with more or less strongly developed 

 ribs. The peristome is simple or double, sometimes re- 

 flected ; the base is generally furnished with a prominent 

 carina or ridge, formed by the peristome. The operculum 

 is horny, hardly spiral, with a basal nucleus. 



Animal with a small foot, against the end of which 

 rests the operculum when the animal is withdrawn ; the 

 tentacles are short, acute ; the snout is extended beyond 

 them as much as the whole length of the animal. The 

 shell is carried horizontally. The other characteristics of 

 the genus are the same as those of the order and suborder. 

 The animal lives in close proximity to the sea. 



