ISO ROBERT E. COKER AND THADDEUS SURBER. 



shelled. These two species seem almost to intergrade (inform of 

 shell) so that one sometimes hesitates in the identification of a 

 specimen of intermediate form. On the other hand, the glo- 

 chidia of these two species show a striking contrast. The glo- 

 chidium of gracilis (Figs. 2 and 20} is oval-pear-shaped and very 

 similar to that of ligamentinus or ventricosus, or to what we 

 generally regard as a typical form; while that of l&vissimus (Figs. 

 I and la) is of the "axe-head" type. Comparison of Figs. la 

 and 2<3 may suggest, however, that the two species are not so 

 dissimilar as at first appears, and Icevissimus may be thought of 

 as intermediate betw r een alatus and gracilis. 



The remarkable fact, yet, is that capax should have this type 

 of glochidium. Capax has always been grouped with ventricosus 

 and ovatus, but ventricosus, at least, has a glochidium of the usual 

 form. Whether the greater significance be attached to resem- 

 blances in larval form or in adult form, it is a suggestive revelation 

 of the adaptability of fresh-water mussels and the inadequacy of 

 superficial diagnosis. Undoubtedly in the classification of fresh- 

 water mussels an entirely undue stress has been laid upon 

 characters of a superficial nature. The ultimate system will be 

 based on a much more thorough analysis of the actual anatomy 

 of mussels in young and adult stages than has ever been at- 

 tempted in systematic work on this group. We have not yet a 

 sufficient knowledge of the comparative anatomy of glochidia 

 to judge of their relative value in the study of relationships, 

 but certain considerations derived from the present case are 

 sufficiently striking to be recorded for their suggestive value. 



Ventricosus is certainly closely related to ligamentinus and 

 luteohis, as judged by the external form. The glochidia, too, 

 are much like those of ligamentinus. The adult form is special- 

 ized most noticeably in being very inflated. Gracilis is special- 

 ized in a different way, being exceedingly compressed laterally 

 and having compressed teeth; Icevissimus (Figs. i& and ic) is a 

 more extreme form of the gracilis type, the teeth are more blade- 

 like, while the shell is equally compressed from side to side and 

 has well-developed wings. The glochidium, while suggestive of 

 gracilis (which is of the "typical" form), is of the "axe-head" 

 form. The adult alatus is of somewhat the same compressed 



