352 Mr. W. Clark on the Littorinidse. 



9. Planorbis trochoides, nobis. 



Testa vix perforata, diaphana, nitida, inconspicue radiato-striata, sub- 

 trochiformi, convexa ; spira parvula, arete convoluta, concava ; su- 

 tura profunde impressa ; anfractibus 3£, penultimo valde convexo, 

 ultimo majori, versus suturam obtuse angulato, extus depresso, in- 

 ferne acute carinato, intus laminis rarissimis plerumque muuito, 

 subtus planato, medio, versus umbilicum inconspicuum, leviter ex- 

 cavato, versus periphseriam vix declivi ; apertura obliqua, sub- 

 sagittata, margine superiori arcuato, prominente, inferiori rece- 

 dente. 



Diam. 3 mill., axis vix 2. 



PL trochoides, Benson, Journ. As. Soc. vol. v. p. 742-3. 



Hab. in hortts palatii proregalis apud castra Barrackpore Bengaliae. 

 Specimina pauca detexi. 



Tbe infrequency and irregularity of the internal laminae in this 

 species, causing them to look more like accidental thickenings of 

 the shell, made me overlook this structure in my original de- 

 scription. The species is singular from the nearly total absence 

 of umbilicus, and from its rendering nugatory, as far as this 

 species is concerned, the characters of the genus " anfractibus 

 omnibus utraque conspicuis," and "apertura ab axe remotissima;" 

 but for the reason stated in the work referred to, its removal from 

 the genus Planorbis appears inexpedient, and the subsequent 

 discovery of a species, proximate in this respect, in PL Sindicus, 

 confirms my formerly expressed opinion. 



It is desirable to observe, that in all the above descriptions I 

 have treated the shells as practically dextral, with reference to 

 the true position of the animal contained in them. 



April 10th, 1850. 



XXXII. — Observations on the Littorinidae. 

 By William Clark, Esq. 



To the Editors of the Annals of Natural History. 



Gentlemen, Norfolk Crescent, Bath, April 2, 1850. 



I offer for the consideration of malacologists a tolerably com- 

 plete memoir on the true Littorinidce, ; that is, I have given de- 

 scriptions of the types of each genus. This account was written 

 during the summer months of the last year, 1849, after a pro- 

 tracted examination of many hundreds of, I may say, nearly all 

 the varieties of Littorina rudis, which have long occupied a posi- 

 tion as species, to which it will appear they are not entitled. In 

 close connection with this subject is a very short correspondence 



