224 GUPPY : MOLLUSCA OF TRINIDAD. 



tions and shape also are somewhat different, the large shells 

 being more ventricose ; but I see no reason to regard them as 

 being other than forms of Ph. rlvalis. The shell so called by 

 DlUvvyn was the Bulla fontinalis of Linne, and the latter de- 

 nomination is retained for it, so there does not appear to be any 

 reason why Maton and Racket's species — the Physa sowerbyana 

 of D'Orbigny included — should not keep the name oi rivalis. 

 Ph. rivalis is found throughout the West Indies and has a 

 wide distribution on the South American continent. I am of 

 opinion that the Physa acuta of D'Orbigny (not of Draparnaud) 

 is also referable to the same species. 



40. Amnicola candeana Orb. 

 Paludestrina candeana D'Orb., 'Moll. Cuba,' vol. ii., p. 9, pi. x., 



f. 13—14- 

 P. auhaiana D'Orb., I.e., p. 8, pi. x., f. 6 — ^7. 

 Bithinia spiralis Guppy, 'Ann. and Mng. Nat. Hist.,' 3 sen, 



vol. xiv., p. 244. 

 P. spiralis Crosse, I.e., p. 53, No. 45. 



There is some question here again as to the proper 

 generic designation of the group. 



In my original description quoted above (May, 1864) I 

 remarked on the variability of our shell. I regard it as a typical 

 and easily verified instance of a mollusc whose characters admit 

 of several species being made out of one. Having had numbers 

 of it in my aquarium I can assert that a single brood may 

 contain all the various modifications from a smooth shell to 

 one to aculeate spines. There are many analogous cases, and 

 I may call to mind that of the Vitrinellas ( = Adeorbis) of the 

 West Indies, etc., which are given as several species, all 

 probably reducible to cne, and the true idea of a species may 

 be regarded as that of a number of individuals such as might be 

 all derived from a single brood, (not, which have all at some 

 time or another been derived from one brood), and all the 

 forms of which might be reproduced from a single pair. 



J.C., vii., July 1893. 



