240 PNEUMONOBRANCHIATA. 



conic manner, the volutions hardly raised, slightly 

 striate longitudinally, and crossed with more 

 remote transverse ones, like the facets of cut 

 glass ; aperture oval, covering nearly half 

 the shell, often chocolate-brown and glossy in 

 the inside, sometimes rosy about the pillar, 

 where the peristome is spread and glossy, 

 forming a slight umbilicus. 



These shells vary very greatly in size ; in their co- 

 lour, from pale brown to dark violet-brown, and 

 especially the colour of the throat, which is rarely 

 bright violet-brown ; in the thickness of the substance 

 of the shell ; and in the shape, occasioned by the dif- 

 ferent degrees of the ventricoseness of the whorls. 



The smaller specimens often have their tips trun- 

 cate. (See p. 231.) Mr. Alder thinks that var. /3. of 

 Mr. Jeffreys, which is found in rivers, frequently in the 

 tide-way, and never has the size of those found in 

 ponds, is intermediate between L. palustris and L.fos- 

 sarius. Probably, the small size is produced by the 

 current not allowing the animal its usual rest ; we often 

 regard a different habitation as a proof of a difference 

 in species, while it may be the cause of the variation. 

 The eggs are like those of L. stagnalis. ( See Pfeif- 

 fer, t. 8. f. 18.) 



92. 5. LimntEus truncatulus. Ditch Mud Shell, (t. 7. 

 f. 108.) Shell oblong-oval, pointed, brittle, per- 

 forated, with six or seven rounded and deeply 

 divided volutions, striolate longitudinally and 

 across ; mouth ovate-oblong. 

 Limneus minutus. Drap. p. 53. t. 3. f. 5, 6.; Al- 

 der, 1. c. 115. 



