76 NILS HJ. ODHNER, STUDIES ON RECENT CHAMIDAE. 



Taxonomical remarks on the genus Ghama. 



It might perhaps be possible to base a classification of the species of Chama 

 on the shape and sculpture of the nepionic shell as well as on anatomical charac- 

 teristics, but hitherto we have very inadequate knowledge with regard to these things. 

 Until more complete knowledge is gained, the shell alone is available for taxonomic 

 purposes. There exist, however, few genera among the Mollusca where so great a 

 polymorphism of the shell is to be found as in Ghama and hence it seems to be an 

 impracticable task to make a delimitation and a classification of the species. 



Anthony (1905) has drawn attention to two different modes of attachment 

 prevailing among the Chamidae. He describes them in the following words: »La 

 fixation peut se faire suivant deux modes, soit par la region antérieure de la coquille 

 seulement, comme chez les Chama riippelli Reeve, et les Chama iostoma Conr., par 

 exemple, et alors le plan sagittal est obliquement incliné sur le plan de fixation; 

 soit par toute la surface de la valve comme chez les Chama brassica, Reeve, par 

 exemple, et alors le plan sagittal est paralléle au plan de fixation . . . Dans ce der- 

 nier cas, et lorsque le plan sagittal est a peu prés horizontal, le contour valvaire 

 est arrondi. Dans le premier cas au contraire . . ., le contour valvaire, au lieu d'étre 

 arrondi, présente une forme allongée dorso-ventrale avec pointe ventrale correspon- 

 dant au sommet de 1'angle diédre constitué par les regions antérieure et posté- 

 rieure. » 



The mode of fixation would offer a good basis for classification, if only it were 

 constant in the same species. This is, however, far from the case. In their most 

 extreme perfection the modes of attachment are apparently profoundly different, but 

 in many species a more or less perfect fixation may offer degrees of transition be- 

 tween these or similar extreme conditions. In Ch. gryphoides, for example, the under 

 valve may be attached to shells with its foremost half; this is flattened and the 

 postenor part is then raised perpendicularly from the base, thus forming an angle 

 with the front portion. When the mussel is attached to corals, twigs and similar 

 narrow things, the substratum has of ten not the same influence in flattening the 

 front part of the shell, but this keeps its convexity throughout, without forming 

 any angle. The same modifications are to be observed in other species, for example 

 Ch. sarda and Ch. reflexa. What part the substratum plays with respect to the 

 form of the shell is exemplified in a striking manner by specimens of the latter 

 species before me. When attached to shells they acquire an ovate or broadly rounded 

 outline with a sharply raised posterior half, but in some specimens which were found 

 attached to a piece of a branchlet the shell has been elongated in the extension of 

 the wood. Such a variation of form caused by exteriör conditions is very common 

 among the Chamidae and has given rise to a great many »species» which are merely 

 modifications in shape of a few real species. I have previously (1917) pointed out 



