15* 



MOLLUSCS, 



African form, has a reversed shell like Physa, but is distinguished by having a 

 tooth or fold on the columellar margin of the aperture. In Chilina the shell is 



dextral, like that of Limncea, 

 but differs in being covered 

 with a periostracum, and ex- 

 hibiting reddish wavy colour 

 markings. The columella is 

 thickened and furnished with 

 one or more folds or plaits. 

 They are found only in clear 

 running streams of South 

 America. 



The curious pulmonate 

 known as Amphibola some- 

 what resembles a periwinkle 

 in form. It lives between tide- 

 marks in brackish or salt 

 water, on mud -flats at the 

 mouths of rivers in New 

 Zealand, and is used as food 

 by the natives. It is abundant 

 in some places, and is a slug- 

 gish creature, subsisting upon 

 the vegetable matter contained 

 in the mud, large quantities 

 of which it passes through the 

 alimentary canal. Professor 

 Hutton says that it will live 

 for a week or ten days in fresh 

 water, and more than a fort- 

 night in salt water, without 

 being exposed to the air. The 

 breathing-orifice is situated on 

 the right side of the neck, and 

 the radula shows some affinity to that of Physa. The shell is solid, globular, with 

 a short spire and an oval aperture. The animal is furnished with a thin horny 

 subspiral operculum. 



ramshorn snail (Planorbis corneus). 



The Hind-Gilled Group, — Order Opisthobranchiata. 



The Opisthobranchs form the second of the three main divisions of the 

 gastropods, and are all marine forms, having the sexes united in each individual, 

 and breathing chiefly by gills or branchiae This character at once separates them 

 from the Pulmonata, and the different positions of the branchiae, and their herma- 

 phrodite nature, serve to distinguish them from the Prosobranchia, the third and 

 last main branch of the Gastropods. In the Opisthobranchs the branchial veins as 



