FAMILY LITTORACEA. 181 



conviction that there is a pulmonary as well as a branchial chamber 

 in Assiminea, and it is certainly the habit of that mollusk to live as 

 mnch out of water, always, however, in its vicinity, as in it. In 

 Valvata, the branchise take the form of a plume of filaments, and 

 are external. In Nerita they form a long, partially free, acute tri- 

 angular membrane, and are internal. 



The branchiate, or gilled, Operculated Cephals, are comprised in 

 three Families : — 



1. Iiittoracea. Head produced into a ringed muzzle. Branchias 



lamellar, internal. 



2. Peristomata. Head produced into a proboscis, tentacles 



sometimes filiform, sometimes cylindrical, with the eyes 

 sessile or pedunculated, on swellings at their outer base, 

 branchise mostly lamellar or filamentary and internal, some- 

 times plumose and external. 



3. Weritacea. Head short, tentacles slender with the eyes ses- 



sile at their outer base. Branchias internal, in the form of 

 a long acute triangular membrane. 



Family I. LITTORACEA. 



Head produced into a ringed muzzle. Branchice lamellar, internal. 



The Littoracea, or Periwinkles, are a family dwelling on the 

 margin of the sea, between tide-marks, or within reach of the spray. 

 They are moderately numerous in species and plentiful in indivi- 

 duals. With comparatively little speciality of geographical distri- 

 bution, they fringe the shore throughout both hemispheres, marking 

 the rocks with riband-like zones ; and the zones indicate the rest- 

 ing-place of particular varieties of species at particular heights of 

 the tide. Some of the Littoracea extend their habitat into brackish 

 water, mud, and swamp, and the respiratory apparatus is modified 

 to the necessities of such a change of existence. In Borneo, Lit- 

 toracea dwell in the mangrove-trees, not only about the roots but 

 among the branches. In Britain we have an abnormal member of 

 this family inhabiting, in countless numbers, the banks and inlets of 

 the Thames, sometimes in the water, but more frequently out of it, 

 as high up as Greenwich. It is the only British species of the 

 family that is not absolutely marine. 



The genus established for its reception is : — 



