254 Mr. W. Clark on some undescrihed Animals 



XXIII. — On some undescrihed Animals of the British Rissose. 

 By William Clark^ Esq. 



To the Editors of the Annals of Natural History. 



Gentlemen, Norfolk Crescent, Bath, Sept. 1862. 



It is stated in the ' Annals/ N.S. vol. viii. p. 48, that I had 

 examined some unrecorded animals of the British Rissoce, and 

 when the minutes were reduced I would communicate the result ; 

 I now fulfil tliat engagement ; and to show that an account of 

 these minute species is considered a desideratum, I need only 

 quote the learned authors of the ' British Mollusca,' who, speak- 

 ing of the Rissoa striatula, remark, " The animal of this, as of 

 too many other Rissoa, is yet unknown.^' The following ob- 

 servations were taken in 1851, but in the present summer I 

 have reviewed, at Exmouth, the several species alluded to, and 

 added some new ones; I may therefore speak with increased 

 confidence of their descriptive accuracy as far as regards the 

 external organs, but I apprehend that a correct anatomy of such 

 minute creatures is a vain expectation ; we must therefore rely 

 on analogy, for at least the general characters of their interior 

 organization. 



It is necessary to mention that the almost microscopic organs 

 of these diminutive species require the aid of good glasses to see 

 their true forms and attributes ; the present descriptions are the 

 result of the organs being viewed through Coddington lenses of 

 as high powers as were consistent with distinctness : inferior 

 means give false appearances, and are often the cause of discre- 

 pancies between observers of the same animal. 



Rissoa striata, Montagu. 



Animal inhabiting a white shell of 5-6 rather tumid, semi- 

 plicated, spirally striated volutions ; it is hyaline white in most 

 parts ; an exception is the upper and under surface of the rostrum 

 and buccal fissure, which are of a sordid light red brown. Mantle 

 even with the shell, except that a minute cirrhal filament, very 

 difficult to be seen, issues from it at the upper angle of the 

 aperture, as in the type R. parva, in which it is never absent. 



The head is a long flat muzzle deeply grooved above and 

 below, with minute lappets on the upper surface near its termi- 

 nation, and on the march is carried a little in advance of the 

 foot : the tentacula are moderately long, divergent, strong but 

 flattened, very little setose ; they do not attenuate to points like 

 the type, but are of the same breadth throughout, and of opake 



