392 Mr. W. Clark on the Littorinide. 
9. Planorbis trochoides, nobis. 
Testa vix perforata, diaphana, nitida, inconspicue radiato-striata, sub- 
trochiformi, convexa; spira parvula, arcte convoluta, concava ; su- 
tura profunde impressa ; anfractibus 35, penultimo valde convexo, 
ultimo majori, versus suturam obtuse angulato, extus depresso, in- 
ferne acute carinato, intus laminis rarissimis plerumque munito, 
subtus planato, medio, versus umbilicum inconspicuum, leviter ex- 
cavato, versus peripheriam 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. 
fab. in hortis palatii proregalis apud castra Barrackpore Bengaliz. 
Specimina pauca detexi. 
The infrequency and irregularity of the internal laminz 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. 
XX XII.— Observations on the Littorinide: 
By Wix11am Crark, Esq. 
To the Editors of the Annals of Natural History. 
GENTLEMEN, Norfolk Crescent, Bath, April 2, 1850. 
I orrer for the consideration of malacologists a tolerably com- 
plete memoir on the true Littorinide ; 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 
