364 



VALLONIA COSTATA + EXCENTRICA. 



By LIONEL E. ADAMS, B.A. 



(Read before the Society, June 13, 1906). 



It will be seen at a glance that the specimens, which are herewith 

 submitted for inspection, bear the same relation to V. costata that 

 V. excentrica bears to V. pjilchella ; that is to say that they are of 

 the ribbed costata form with a very pronounced eccentricity. I 

 picked them out of a number collected round Kettering ; unfortun- 

 ately, as they were long since dead, it was too late to submit them 

 to an expert in radulae for examination. The eccentricity of these 

 specimens, as also of V. excentrica, seems to arise in the following 

 manner : — Towards the end of the second year's growth the shells of 

 some individuals expand slightly more than usual, though not assum- 

 ing the expanding trumpet-mouth of the adult of three years' growth ; 

 then the animals hibernate, and the next spring, on emerging from 

 winter quarters, drained of much of their vigour, are obliged to 

 continue the shell-growth on a more economical scale, and the con- 

 tinuation of the outline of the shell is accordingly contracted in 

 the process. 



We find this same eccentricity occasionally in Platwrbis corneiis 

 Linne, and always in the case of Gorilla erronea Alb., and C. odonto- 

 phora Benson. A similar contraction of the last whorl is common in 

 Limncea peregra Miill., and L. stagnalis Linne, but in these cases 

 the eccentricity of outline is not noticed as in discoid shells, which 

 are viewed and figured from a different point of view. I think, too, 

 that the peculiar distortion of Gibbulina lyonetiana Pallas is due to 

 the same cause, and doubtless many similar examples will be familiar 

 to most conchologists. 



Now, in the case of the foreign examples mentioned, the irregu- 

 larity of outline is, as far as I am aware, universally characteristic ; 

 but if it were not, and symmetrical individuals were found to 

 exist, would these be recognised as worthy of separate specific rank ? 

 So far, the occasional eccentric Planoibis cortieiis have found no 

 advocate for even varietal differentiation. It would, of course, be 

 very unscientific to cavil at the value of specific differences, merely 

 because of the minute size of such objects as the VallonicE, but it 

 would be reassuring to have some particulars of the intermediate 

 forms, if such there be. Tryon ("Manual" (2) vol. 8.), describes eight 

 species in the '■'■ pulchella group," and in the " costata group " five 

 species with four varieties of costata. The radula of V. pulchella is 

 described as having 65-68 transverse rows of 27 teeth, and that of 



