Mr. W. Clark on the Rissoa rubra. 109 



tudinal furrow about the middle, which forms a raised rib ou the 

 under part, the whole of that area being thick, coarse and irre- 

 gular, with, at the nucleus, which is nearer the base than the 

 centre, a long, strong-pointed testaceous apophysis, more pro- 

 minent but not so medial as in Jeffreysia, but stronger and 

 longer ; indeed as much so as in some of the Chemnitzice. 



These animals inhabit the lower littoral levels at Penzance, 

 their locomotion is deliberate, and they evince considerable shy- 

 ness. There are many fasciated varieties and a white one. 



This animal nearly approaches the littorinidan group, and 

 conducts fi'om Rissoa to Jeffreysia : as the latter and it have 

 analogous subtestaceous opercula and apophyses, they naturally 

 lead to the Pyramidellida. But this species cannot be placed in 

 Rissoa on account of the singular operculum, as the like is not 

 seen in any other species of that genus, and for many other ani- 

 mal discrepancies. Philippi unaccountably omits all mention of 

 the principal pecuHarity, the curious operculum, but he does say- 

 that the animal departs somewhat from those he has examined, 

 both as regards the organs and the shell ; and I add, that with 

 the exception of the verv' short muzzle and depressed hne in the 

 after-part of the foot, there is not another external organ that 

 has much concordance with the typical Rissoa. 



Neither can it be associated with Jeffreysia, which, indeed, 

 agrees with it, essentially, in respect of the operculum, but the 

 animals of the two are very different. I shall therefore pro- 

 pose for it a new genus, which ought, I think, with Jeffreysia, 

 to form a family intermediate to the Littorinida and Pyrami- 

 dellida. I have omitted to mention that M. D'Orbigny's sub- 

 genus Rissoina cannot receive it, as with a testaceous operculum 

 and apophysis, it is of the spiral or littorinidan tj-pe, whilst the 

 present object is of subannular elements ; and I consider the 

 operculum, though so much neglected, to afford a most important 

 generic and differential diagnosis; but independent of these 

 points, I could not, agreeably to my views, accord with such an 

 allocation. I repudiate all subgenera, which I consider as an 

 awkward attempt to define what is undefinable — an intermediate 

 condition between a genus and a species. I think, when a species 

 is so discordant with the generic tj'pe, that it ought to merge 

 elsewhere, and take on a substantive capacity and become the 

 t\^e of a new genus ; but there can be no objection to the term 

 sub when used adjectively to qualify a word, as subannular, sub- 

 rotund, and subsymmetrical, &c., but not substantively, as then 

 it becomes the source of innumerable absurdities; therefore 

 with me a genus has no intermediate state beyond species and . 

 their varieties. I have mentioned these views in the last para- 

 graph of a former paper in the ' Annals,' vol. vi. p. 29. N. S. 



