122 Mr. W. Clark on the Aclis unica. 



meso-branchial lobes, and the posterior branchial lobes, which 

 are strongly depressed ; surface strongly and closely granu- 

 lated irregularly, the granules being of three or four different 

 sizes ; a few small tubercles like those on the anterior margin 

 also occur, one being conspicuous on each side, slightly below 

 the middle of the furrow which separates the proto-gastric 

 from the hepatic lobes, and two on each side on the meso- 

 branchial region a little within the middle. Width of cara- 

 pace 10 lines, length 5^ lines, transverse length of one orbit 

 1^ line, depth of middle of hepatic region 3 lines. 



The coarsely granular surface easily distinguishes this species 

 from the smooth and glossy B. Buchii, Reuss sp. The anterior 

 lateral margins are also strongly curved. There is a third spe- 

 cies in the University collection at Cambridge, distinguished 

 from the other two by a very fine uniform granulation of the 

 surface, but it is not sufficiently perfect for description ; it might 

 be called R. granulosa. 



Rare in the Upper greensand of Cambridge. 



{Coll. Mr. Carter, Cambridge.) 



XII. — On the Aclis unica, Auct. By William Clark, Esq. 

 To the Editors of the Annals of Natural History. 



Gentlemen, Exmouth, 29th June 1854. 



I propose, with your permission, to give an account of a very 

 rare mollusk which I discovered this day, and which has hitherto 

 evaded, in a living state, all our researches ; I have sought it for 

 thirty years, and may therefore sing " Io Pseans " with the illus- 

 trious author of the ' Amorum/ as at last, as with him — 



" Decidit in casses prseda petita meos." 



Let this instance of unexpected success impress on us the value 

 of the " nil desperandum." The discovery of this creature has 

 long been a desideratum, as it will solve several malacological 

 questions : it has from Montagu's time run the gauntlet of 

 nearly all the genera, agreeably to the conchological surmises of 

 naturalists, of whom scarcely two are in accord, and all in error ; 

 as my notes require me to place it in a position it has never yet 

 occupied, and which I believe will prove to be its true malacolo- 

 gical status. Our ignorance of every circumstance attendant on 

 this almost microscopic being has invested it with a strange 

 diversity of position and consequent structure, but the light of 



