Mr. W. Clark on the Skeneadse. 47 



type, instead of R. parva, and the other for the "planorbis," 

 which is equally a Rissoa of another form ? 4ffc 



I have scarcely a doubt that the Adeorbis subcarinatus, from the 

 aperture being of the same character as the Trochus serpuloides, 

 T. Cutlerianus and T. nitens, will, when discovered, turn out a 

 Trochus in all essential characters, even if it has a testaceous 

 operculum like its near relative Phasianella pullus. Surely the 

 British list may with advantage be relieved from the superfluous 

 genera Cerithium and Skenea, and ultimately probably from Ade- 

 orbis. Is it to be contended, that because an animal has an 

 elongated shell of twelve volutions, and another a discoidal one 

 of three, it cannot be a Rissoa, and that such a departure from 

 the type demands that the genus Cerithium be constituted for 

 the one and Skenea for the other ? I would ask, what is the 

 classic number of volutions which stamp the Ilissoidean ani- 

 mal ? It may be said that the so-called C. reticulatum has a 

 canal at the base of the aperture ; this is scarcely so ; it is a 

 mere contraction and attenuation at that part, giving an effuse 

 aspect ; the mantle is even with the shell, without a canalicu- 

 lation : many of the Rissoa have these parts quite as much de- 

 veloped. Again, it is said that its operculum and that of the so- 

 called Skenea planorbis are suborbicular : I say, not more so than 

 some of the Rissoa ; and both these animals have very much the 

 same paucispiral rapidly increasing character of the opercular 

 increment as in the Littorince. I think that the C. reticulatum 

 and S. planorbis differ less from the Rissoidean type, the parva, 

 than any other of the Rissoa admitted by authors into that ge- 

 nus. If these positions are not admitted, we ought, to be con- 

 sistent, to manufacture a separate genus for every petty variation 

 of each Rissoa, and expunge the term ' species ' from the mollus- 

 can vocabulary. 



I am, Gentlemen, your most obedient servant, 



William Clark. 



Exmouth, 3 p.m., June 3, 1851. 

 P.S. — I have great satisfaction to say that whilst I am writing, 

 I have under examination the so-called Skenea Cutleriana, disco- 

 vered alive within the last half-hour. The animal at first view 

 exhibits the general characters of Trochus serpuloides ; the only 

 differences between the two are, that in the now Trochus Cut- 

 lerianus the cilia of the tentacula and vibracula are less close- 

 set, the curved auricles of the foot much flatter and broader, 

 the foot both anteally and posteally more rounded, and the eye- 

 prominences may almost be called very short pedicles. The ani- 

 mal is infinitely more active, exhibiting a three or four times 



