MARSHALL : TAPES GEOGRAPHICUS. 167 



only one of not less than lialf-a-dozen distinct forms of shape and 

 coloration, each " readily separable " from the other. 



Similar circumstances attend T. pei-fo>ans, which the author 

 separated from his T. puUasb-a as a distinct species, and it was 

 so held until Professor Forbes disputed its specific identity and 

 reduced it to a variety, which has been fully accepted. One of 

 its chief peculiarities is tliat although usually colourless, a specimen 

 now and then occurs which has the posterior end more or less 

 marked as in T. puUastra. So much for coloration ; while as re- 

 gards shape, the form perforans (as well as plag'id) is still further 

 removed from pullaslra than even geo;faphicns, with the more dis- 

 tinctive attributes of different habitat and sculpture. Compare also 

 var. oblonga Jeffr., which in shape connects the two forms. 



Mr. Woodward mentions two other points, though I do not 

 gather how far he urges them as essentials — (i) "the hinge-plate 

 is not so prominent " (he might also have added the teeth), 

 "sloping back more into the valve." This is simply degree of 

 development between a solid shell and a thinner one; precisely 

 the same differences will be found between a thick and a thin 

 Cardium edule, Mcxdra solida, or other bivalve ; (2) " the posterior 

 portion of the hinge has a strong purplish tinge." This will not 

 bear the test. If a fair number of specimens be examined, it will 

 be found that, in both forms, not only the position, but the shape, 

 size, and even presence of these internal stains are of the most 

 capricious character. 



I remain firmly convinced that T. pullastra is the same species 

 as T. geographicus, and that Jeffreys and others were right in com- 

 bining them. In my point of view, T. pullastra is the parent 

 stock of this highly plastic species, and has thrown off various 

 varieties ot shape and colour, of which that oi geographicus happened 

 to be described first. 



A FINAL NOTE ON TAPES GEOGRAPHICUS. 



By B. B. woodward. 



(Read before the Society, March 19, 1902J. 



In reference to Mr. Marshall's paper, which the Secretary has given 

 me the opportunity of seeing, I can only add that the point of the 

 whole case seems to be, are Mr. Marshall and I talking about the same 

 thing? This can only be proved by the production of his specimens 

 as suggested. Reading l)etween the lines it begins to seem as if Mr. 

 Marshall were unacquainted with the true Tapes geographiais — at 

 least under that name. 



