80 LIMN.EID.E. 



2 to 5 rows of curved plates, which are arranged on each side 

 across the spire, opposite to each other and at nearly equi- 

 distant intervals. L. 0-065. B. 0-2. 



Habitat : Slow streams and ditches in the home and 

 eastern counties, as well as in Guernsey (Lukis), Notts 

 (Lowe), and Co. Tipperary (Humphreys) ; but it is a 

 local species. It is found in our upper tertiary strata. 

 According to Malm it occurs in Sweden ; and it appears 

 to have a wide range over the more temperate parts of 

 Europe, as far south as Toulouse. 



This pretty and curious little mollusk is rather active, 

 and is usually found on duck-weed and other aquatic 

 plants. It lays only from 3 to 8 eggs, which come to 

 maturity in ten or twelve days. The internal plates, 

 which are observable outside the last whorl of the shell, 

 form half-closed chambers : and the animal retreats into 

 the front one for safety. These plates appear to answer 

 the same purpose as the teeth or folds which barricade 

 the apertures of many of the small land-snails. They are 

 also only formed in adult individuals. The peculiarity 

 of this structure induced one of its earliest discoverers, 

 Lightfoot, to call the present shell a Nautilus ; and the 

 late Professor Fleming proposed for it on the same grounds 

 a new genus [Segmentina) , which has been adopted by 

 Capt. Brown and Dr. Gray ; but, even as a test of concho- 

 logical distinction, this character does not seem to be of 

 any value. Miiller noticed, in his description of the next 

 species, that a few of the specimens which he had exa- 

 mined had two streaks, like ligaments, in the upper part 

 of the last whorl, apparently indicating the marks of 

 fracture and mending of the shell, by which he may 

 possibly have meant this species ; but he only described 

 one species of this form, which still bears the name he 

 gave it, Plano7'bis nitidus. The present species was first 



