204 LECTURES ON MOLLUSCA. 



Australian Periwinkles are top-shaped and ash-colored : they were 

 first named Risella. Some species, living in marshes of brackish 

 water both in England and the East Indies, instead of having the eyes 

 on the base of the tentacles, as in all others of the tribe, have them 

 on the tips ; or rather perhaps on eye stalks joined to the tentacles. 

 They are called Assiminea. In Tectarius, the shell is top-shaped, 

 strong, and rudely knobbed outside. Echinella is intermediate between 

 this form and the true Periwinkles ; with knobbed exterior, often a 

 lump on the pillar, and a many-whirled operculum. Modulus has 

 also a many-whirled operculum: it is flatly top-shaped, with a deeply- 

 cut tooth at the pillar. Fossarus differs from the Periwinkles in hav- 

 ing little frontal lobes between the tentacles. The habits of the ani- 

 mal, as well as the shell, greatly resemble Narica. A few species 

 from the west coast of America have a lump on the pillar, and are 

 called Isapis. Shells closely allied to Periwinkles have been found in 

 the Oolitic rocks. In the newer tertiaries, the present species are 

 found, even with color bands ; and with shells curiously distorted (as 

 now in the Baltic) from the too large admixture of fresh water. 



Family Lacunhle. 



This little tribe of northern shells differs from the Periwinkles, 

 (which the shells greatly resemble, except that they have a chink in 

 the pillar,) in having no jaws. Dr. Gray even assigns to them a 

 proboscis. There are two little tails behind the operculum as in 

 Rissoa. The Lacuna vincta is common in the New England seas, and 

 deserves a careful dissection. There is no siphonal fold in the mantle. 



Family Planaxid;e. 



The shell of Planaxis differs from Litorina in having a sharp notch 

 in the pillar, through which protrudes a small breathing pipe. The 

 creatures are all tropical, and are extremely plentiful where they live. 

 One little species is remarkable as being common both to the West 

 Indies and the Red Sea. They have a solid, stumpy foot, and a long 

 snout. In Quoyia, there is a curious sharp keel running along the 

 pillar. The shells of this family are often remarkable for the great 

 difference in appearance between the young and the adult state. This 

 is peculiarly the case in the little Rissoid shells called Alaba, of which 

 two extremely similar species are found in tropical America, one in 

 each ocean. They would scarcely be distinguished when adult ; but 

 the sculpture of the nuclear snout at once separates them. The oper- 

 culum is half-mooned shaped and slightly spiral. 



Family LiTiOPiDiE. {Gulf-weed Snails.) 



The Litiopai are tiny shells, very like Planaxis, but the animals 

 have a curious series of lappets on each side of the mantle, as in the 

 Top-shells. They travel over the ocean on the gulf-weed, from which 

 they suspend themselves by spinning glutinous threads. If they lose 

 their hold, they make a bubble which they send up to find the weed 



