1892.] 



THE BRITISH NATURALIST. 



73 



than 100 palaearctic Rissoas in his own collection. Woodward says 

 that there are about 100 fossil species. No doubt careful collecting 

 will add enormously to the total, — last year, e.g., '''St. Helena 

 contributed a substantial item of new species, I forget exactly how 

 many, but over a dozen, and Australian waters are not unprolific 

 according to Mr. Petterd.f As elsewhere, the " splitters" have been 

 busy, and not altogether unsuccessfully so, in this genus of dwarfs. 

 As we are concerned with British Rissoas only, we v/ill take singly the 

 divisions or sub-genera which have British representatives, in no 

 particular order, but according to Canon Norman's classification of the 

 species, though, by the bye, he gives all these sections generic rank. 

 It should be premised that our small friends were originally classed 

 by Gray with the periwinkle family, but are now justly elevated to 

 the rank of a separate one called RissoidcB. 



Sub-genus Onoba, Adams, — This contains 3 species of which two, 

 R. vitrea, Mont, and R. pvoxima, Alder are rare deep-water species and 

 may be passed over, but the third, named R. striata, Adams, is of 

 commonest occurrence both alive and dead ; for live specimens search 

 the roots of small seaweeds, especially the Coralliiia ojicinalis, above 

 low-water mark. A taste of mud alv/ays seems to suit it, and 

 frequently the undersides of stones, resting rather firmly on a muddy 

 sand will produce a good harvest. For distinguishing marks we have 

 a good guide, both in colour and form ; the former is usually quite a 

 dull white, with two broad indistinct red bands on the last whorl only. 

 These always end before reaching the edge of the mouth. Dead shells 

 from shell sand will show these bands up more sharply. The shell is 

 3 or 4 millimetres long (about ^ inch) and cylindrical, with a blunt 

 spire — there is very little difference all the way up. Some curious ribs, 

 fairly obvious, run from the suture (or joining of the whorls) down- 

 wards, halfway across each whorl. Note then the habitat first of all, 

 and examine your weed-washings for a small oblong white shell, blunt 

 and hardly tapering, perhaps recalling in miniature some of the 

 proportions of Cochlicopa hibrica. Curiously twisted fellows are not 

 uncommon especially in dredgings, and have been dubbed nionstr. 

 distortum. There is the usual variation of a smooth [i.e., ribless) form, 

 mostly of northern occurrence, called var. arctica, Loven, probably 

 identical with the North American R. ciculeus, Gld. Just a word on 

 tlie best way of working the small seaweeds in quest of minute shells. 

 It is hopeless to do so at all thoroughly on the spot, and they should 

 be carefully scraped off close, with the sand or mud adhering to their 

 roots, and transferred to tins or bags - the growths in small rock-pools 

 near low water are most prolific. Carefully pulled to pieces and 



* Proc. Zool. Soc, April, i8gi 



t Q. Jour. Conch., IV., p. 137. 



