Mr. W. Clark on the Chemuitzia opalina and C. diaphana. 293 



specimens of the beautiful Rissoa opalina. Though somewhat 

 torpid, I have observed the organs, an account of which I am sure 

 will gratify many of your readers. There is no recorded descrip- 

 tion of the animal. This excellent discovery is due to the per- 

 severance of George Barlee, Esq., whose laborious and painful 

 journeyings — 



" per omnes 



Terrasque, tractusque maris," 



of the wilds of the Ultima Thule, and Hebridean seas, have en- 

 riched science with so many rare and interesting objects ; the pre- 

 sent one is invaluable, as it clears up several doubtful questions 

 which might long have remained unexplained, if this curious 

 animal had continued to escape observation. 



Rissoa opalina, auct. Chem. opalina, mihi. 



Animal inhabiting a spiral subglobose shell of three volutions. 

 Mantle of the palest azure, slightly canaliculated, otherwise even 

 with the shell. The head is a rounded, short, contractile pro- 

 boscidiform muzzle, which is rarely carried beyond the foot and 

 tentacula j it is vertically cloven at the terminus and under part, 

 furnished with a pair of subcircular jaws and lingual riband, 

 which in several of the examples I frequently saw protruded 

 after the manner of the Rissoce ; the head and neck are brindled 

 with fine dark lead-coloured lines. 



There are a pair of tentacula on each side the neck behind the 

 muzzle, springing from a distinct common origin or pedicle, not 

 formed by the fissure of any part of the head, divergent, very 

 short, thick, very little flattened, of nearly the same size through- 

 out, each pair connate with their respective stamens, very mode- 

 rately setose, quite blunt at their terminations, beneath pale yel- 

 low, above delicately aspersed towards the extremities with pale- 

 coloured veiy minute points } the eyes are large, black, placed 

 very far back on the neck, on, if at all, slight eminences, perhaps 

 immersed in them, apparently in a line with the centre of each 

 tentacular pedicle ; these parts were seen with great difficulty, 

 and only came into view in two examples ; they however may be 

 observed through the paler-coloured shells, but are not exserted 

 on the march beyond the margin of the aperture. The foot is 

 oval in quietude, showing a narrow lead-coloured margin, in 

 action somewhat truncate anteriorly, with very small auricles, 

 posteriorly forming a gradually attenuated termination, without 

 cirrhi, but slightly emarginate in one or two specimens. The 

 corneous operculum is suboval, marked with fine subannular 

 striae, with a small central process, rib and groove, sometimes 

 with two minute raised points or nuclei contiguous to each other ; 



