34 Mr. Alder's Catalogue of Land and Fresh-'water Shells. 



Animal, dark lead-colour above, and white below. 



In moss and under stones ; rather rare. Meldon, Walbottle Dean, and Cullercoats, 

 Northumberland ; and Castle Eden, Durham. 



The internal structure of this shell is very curious and elaborate. It has a raised 

 thread-like lamina running spirally round the columella in the manner of a corkscrew ; 

 and another similar lamina running spirally in the centre of the upper side of the whorls ; 

 opposite to this, across the lower side of the whorls, are set, at short distances, small, flat, 

 testaceous plates, similar in situation to the septa in Segmentina lineata, Flem. (Nauti- 

 lus lacustris, Mont.) This complicated structure is no doubt intended to answer some 

 useful purpose in the economy of the animal, but what are its uses, besides the protec- 

 tion of the animal in a young state, I have not been able to discover. It is not 

 continued through the lower whorls, and is most distinctly seen in young shells. 

 The same curious structure is observable in P. umbilicata, but hitherto it seems 

 to have escaped the observation of conchologists. P. marginata, which in its young 

 state, is so very similar to the young of these two species, as to be otherwise scarcely 

 distinguishable from them, is at once detected by the absence of this complicated struc- 

 ture. It is also entirely wanting in P. edentula and P. sexdentata. The internal struc- 

 ture of P. pygmcea and P. Vertigo I have not had an opportunity of examining. 



32. P. sexdentata. 



Turbo sexdentatus, Mont. ? 



Shell minute, ovate, sub-cylindrical, horn-coloured, very minutely and regularly stri- 

 ated ; with 5 much rounded whorls, the penultimate one generally projecting a little be- 

 yond the rest ; aperture semi-oval, contracted on the outer margin, with 6 teeth, 2 of 

 which are on the body whorl, 2 on the pillar, and 2 on the outer lip ; lip thickened, re- 

 flected, and generally having a rib, or rather swelling of the whorl, behind it ; umbilicus 

 moderately large ; length about Jg^th of an inch. 



In wet moss in woods ; rather rare. 



This species differs from the P. antivertigo of Dhaparnaud in being smaller and with 1 

 tooth less on the body whorl. Dr. Fleming informs us that Montagu confounded two 

 species in his Turbo sexdentatus, and as the present species is perhaps the only truly 

 six-toothed British Pupa, it was probably one of them. I have, therefore, preserved the 

 name as most appropriate, but being by no means certain that it is the shell described 

 by Dr. Fleming under this name, I have thought it necessary to add a description. Dr. 

 Fleming takes no notice of the striae, and says that his shell has from 6 to 7 teeth, 3 of 

 which are on the body lip. On a careful examination of at least fifty specimens, I cannot 

 find that it ever has more than 6 teeth in the mature shell, and 5 in younger ones, 2 

 only being on the body whorl. 



33. P. pggmcea, Drap. 



Under stones in dry situations ; rare. In old limestone quarries at Marsden and West 

 Boldon. 



Readily distinguished from the preceding by being rather larger, without striae, and 

 , with only 4 teeth, 1 of which is on the body whorl. 



