202 LECTURES ON MOLLUSCA. 



Family Helicinid.e. 



This group consists of very pretty compact little shells, which most 

 abound in tropical America, but are also found in the Pacific and East 

 Indian islands. They have half-oval mouths, with an operculum of 

 concentric elements. The teeth are 3* l - 3, as in Litorina. The ani- 

 mal has a propensity to eat away the inner layers of its shell, like 

 Nerita. Helicina has a plain mouth, with a lump on the pillar lip. 

 In the West Indian group Alcadia, there is a slit on the basal lip, 

 and the shelly operculum has a projecting tooth, to correspond with 

 it. In Trochatella, the shell is top-shaped, and there is no lump on 

 the pillar. In LucideUa, the lip is distorted with teeth. Stoastoma 

 has a twisted notch, reflected as in Pupinella. 



Family Aciculid,e. 



A family of very small, turreted shells connects the land with the 

 sea Periwinkles. They have the eyes on the back of the head, behind 

 the Periwinkles, and a very thin operculum, with few whirls, Acicula 

 has the outer lip of the shell plain ; in Geomelania it is produced into 

 a tongue. 



Family Trtjncatellidje. {Looping Snails.) 



These little creatures have a very short, round foot, and a muzzle 

 prolonged into two lappets. They loop on these, like the geometric 

 caterpillers. They have highly organized eyes., behind the tentacles. 

 A peculiarity in Truncatella is that on reaching maturity it drops off 

 its long, slender spire, fastening up the broken part. A little Kissoid 

 shell, called Tonichia, is said to have similar peculiarities. 



Family jEFFREYSIADiE. 



Among the vast group of tiny shore shells commonly called Eissoa, 

 Mr. Alder found some, small among the small, who never draw their 

 eyes outside their houses. They are placed far back behind the tenta- 

 cles, and look through the transparent shells, which float among 

 seaweeds in rocky pools. In Jeffreysia, the muzzle is cleft into false 

 tentacles, as in Ampullaria. In Hyala, it is plain, and the creature 

 has relations with Pyramidellids. The operculum in Jeffreysia is of 

 concentric elements, with a bolt standing from it inside at right 



angles 



Family Rissoid^e. 



Almost on every coast where there are any stones for sea-weed to 

 grow from, there will be found, living among the algre, or dead in 

 multitudes among the sand, a great many species of shells like very 

 tiny Periwinkles, but much prettier in their shape, sculpture, and 

 coloring. They generally have a short, slightly cleft muzzle, joined 

 on to the front of the foot, which is pointed behind. There is a curi- 

 ous little tail under the operculum. The lateral teeth are more claw- 



