MARSHALL : ADDITIONS TO " BRITISH CONCHOLOGY." 227 



but the degree of development depends more on the convexity of the 

 whorls— -the more convex the whorls the deeper the umbilicus. There 

 are several well-marked forms besides those indicated by Jeffreys as 

 varieties. One is small, having the proportions and contour of Eissoa 

 pulcherrima, a short spire and ample body-whorl; another, also dwarf, 

 is like a short cylinder, with turreted whorls, and distinctly umbilicate. 

 Both are from the Channel and Scilly Islands. The Shetland form is 

 broader throughout, the young of which are globose. 



The habitat of O. rissoides is unique and highly interesting. It is 

 nearly thirty years since I first found them gregarious and commensal 

 with Mytilus edulis at Teignmouth ; a few years afterwards I found 

 them under similar conditions at Exmouth, and I have subsequently 

 found this to be general throughout South Devon. I have no doubt 

 they will be found in similar situations elsewhere, though I have not 

 been successful in the search, for dredged and dead specimens 

 occasionally exhibit fragments of the byssi of Mytilus attached to them. 

 It is true they are frequently found in the weeds of rock-pools on most 

 of our shores, but these are small and stunted, and do not attain half 

 the size of those above mentioned. The colonies of M. edulis in 

 which they occur are generally situated about the middle of the littoral 

 zone, and are so densely matted together as not to be easily disin- 

 tegrated; but, an opening once made, the O. rissoides may be seen 

 absolutely swarming underneath in a thin stratum of mud, very often 

 themselves anchored or entangled in the byssi. None of the varieties 

 occur with it, but all belong to the typical form, and attain two lines 

 in length, the largest I have seen, and distorted examples are not 

 uncommon, probably the result of overcrowding. Nor do any other 

 genera of mollusca associate with them except an occasional O. pallida, 

 which is notoriously a commensal species. There is, however, a fair 

 assortment of other marine life associated with them in this curious 

 environment, comprising small Crustacea, annelides, &c. Another 

 curious fact is that the Odostomia do not affect all colonies of mussels 

 alike, nor a tithe of them, but appear to be most capricious in their 

 choice, choosing one patch of mussels and neglecting a dozen others 

 in their vicinity. I have never found them in a bed of adult mussels, 

 but generally where these were half grown. 



Var. alba Jeffr. — 6 to 45 f. Scilly (Smart and others); Lismore, 

 6 f. (Knight)! Sound of Sleat, 45 f. (Somerville and J.T.M.); Sark; 

 Plymouth Sound; Borough Island; Torbay; Connemara and Mayo; 

 Iona, 20 f. ; Aberdeen; Dornoch Frith; the Minch, 45 f. ; Hascosy 

 Sound, E. Shetland, iof. This is very different in aspect to the type, 

 and much smaller; the mouth is more oval, the whorls spindled, and the 

 tooth and umbilicus rather more constant and distinct. It is a scarce 



