1897.] NEW YORK ACADEMY OF SCIENCES. 77 



ADAPTATION OF THE SHELL OF PLACUANOMIA 



TO THAT OF SAXIDOMUS, WITH REMARKS 



ON SHELL ADAPTATION IN GENERAL. 



By Bradney B. Griffin, B, Sc. 



During one of the dredging trips made in the summer of 1896 

 near Port Townsend, Washington, there came into m}^ posses- 

 sion a left valve of a species of Saxidomus, one of the common 

 clams of the Pacific Coast. Upon this valve near its anterior 

 margin was attached a shell of Placuanomia. The latter exhibits 

 an interesting though not uncommon phenomenon ; ordinarily 

 smooth and marked only by indistinct lines of growth or low 

 radial folds, it has here assumed lines and folds that coincide 

 exactly with the concentric markings of the Saxidomus. We 

 have here evidently a case analogous to one recently described 

 by Arnold Graf,* wherein a Crepidula had assumed the 

 radial folds of a Pecten upon which it was attached. As the 

 same principles are here concerned the case well merits a de- 

 tailed description. I have added a rough sketch to aid in 

 elucidating the main features. Many of the intermediate con- 

 centric lines have been omitted to avoid confusing the drawing, 

 and for a like reason those indicated have not been continued 

 the whole length of the shell. 



The Saxidomus in question is marked by frequent sharp raised 

 concentric lines, distributed at irregular intervals. They occur 

 sparsel^Mn the umbonal region, but become more frequent as the 

 margin is reached, where they are quite fine, regular and closely 

 set. The line a and several between a and b not indicated on the 

 drawing are represented on the Placuanomia by faint folds, but as 

 the margin is appi'oached the impressed lines become sharper and 

 more distinct until I is reached. From I to the margin the at- 

 tached shell is corroded and the surface pattern confused, but 

 with careful scrutiny indications are seen of numerous fine lines 

 which correspond with the more marginal ones of the Saxidomus^ 

 The completeness of the correspondence is quite remarkable. Not 

 only the sharper and more distinct lines are copied, but even 

 the more minute concentric strife are impressed ; and, moreover, 

 whenever a line is double or triple, however near the component 

 lines may be to each other, this double or triple condition is ac- 

 curately copied on the Placuanomia. In all cases the distance 

 between the components is greatly exaggerated in the drawing. 



♦Adaptation of the shell of Crepidula fornicata to the shell of Pecten Jacobceus. 

 Trans. New York Academy of Science, April 3, 1896. 



