MOLLUSCS 379 



been observed climbing up and down permanent tlireads of this character for 

 nearly three weeks. It is not used exclusively by the animal that makes it, since 

 three Fhysce and a Limnma have been found on a Physa thread together, and often 

 two Physic will meet on a thread, and either tight for its possession, or pass one 

 another content with the right of way. Among the molluscs living near water 

 is the amber-snail (Succinea putris), the shell of which is about three-eighths 

 of an inch long, and oval in shape, with three or four whorls, the mouth beino- 

 oval and large. From the colour of the shell the species receives its popular 

 name. 



Leaving the air-breathers or pulmonates, we arrive at another order, that of 

 the prosobranchiates, of which there is a well-known fresh-water representative 

 in Paludina vivipara, characterised, as its name indicates, by bringing 

 forth its young alive. It has a greenish shell, banded with brownish red, the 



sp, .;*■ ,* !•**"- 



THE LARGE RrVER-MDSSEL. 



whorls being rather swollen and six and a half in number, with the suture slight 

 and the apex blunt. The shell is about lj inches long, or the same length as 

 another species, P. contecta, often mistaken for the first, but distinguished by the 

 deep suture and umbilicus. In Valvata piscinalis we have a member of the next 

 family, in which the shell forms a broad cone, and the whorls are six in number. 

 As it is only a quarter of an inch in length, this mollusc is often overlooked, 

 though in many places it exists in considerable numbers in ponds and sluggish 

 streams, and particularly in canals. The gill-plume, which is protruded from the 

 round mouth of the shell, has fourteen filaments on each side of the stalk. 

 Pond and River Several kinds of bivalve molluscs also dwell in the inland waters 



Mussels. f Europe. A common species is the one technically known as 

 Sphcemum corneum, which is two-fifths of an inch in length, with a thin, convex 

 shell of yellowish horn colour, having faint concentric bands. Another species, 

 S. ovale, is larger and more noticeable, but somewhat local. Of another genus 

 (Pisiduim) there are several species of small size, the biggest being no larger 



