April, 1911.] Anatomy and Physiology of the I'nionidae. 331 



20. Orton, E. The coal fields of Ohio. Geol. Surv. Ohio 7: 255- 



290, 1893. See also subject index of Ohio reports in BibH- 

 ography of Ohio Geology; Geol. Surv. Ohio, Bull. 6, 1906. 



21. Prosser, C. S. Revised nomenclature of the Ohio Geological 



formations. Geol. Surv. Ohio, Bull. 7, 1905. 



22. Renault, B. Recherches sur les bacteriacees fossiles. Ann. 



des. sci. nat. bot. VIII serie, T II: 275-349, 1896. 



23. Scott, D. H. The present position of palaeozoic botany. 



Progressus Rei Botanicae I: 139-217, 1907. 



24. Ward, L. F. The geological distribution of fossil plants. 



U. S. Geol. Surv., Washington D. C., 1889. 



25. White, D. The upper Palaeozoic floras, their succession and 



range; in Willis, B. and Salisburv, R. D. Outlines of geo- 

 logic history: 138-160, 1910. 



NOTES ON THE ANATOMY AND PHYSIOLOGY OF THE 



UNIONIDAE.* 



V. Sterki. 



The Unionidae are not only the most conspicuous part of otu" 

 molluscan fauna, but also the most interesting. Some of their 

 anatomical and physiological features have come to our knowl- 

 edge only recently, and in few groups of animals, have the last ten 

 to fifteen years brought such radical changes of classification. 

 Up to 1900, the genera were generally based upon the shells: those 

 with complete (regarding the family) hinges were called Unio, 

 those with more or less defective hinges were Alasmidonta, or 

 Margaritana, and those without hinge teeth were Anodonta. Con- 

 chologists generally know that the groups and genera are now 

 established principally on the soft parts, mainly the branchiae, 

 not exactly coincident with the formation of the shells. The 

 branchiae, or gills, of this group of moUusca, have three very 

 diftcrent functions: respiration, nutrition (as food gatherers), and 

 as brood chambers for the ova and embr^^os. 



The general morphology and anatomy of the fresh-water 

 mussels is well known, but the special features are frequently not 

 mentioned, or very fragmentarily, in text books on zoology, and 

 not even in recent special works on mollusca. Our lowest form, 

 at least in one group, and in one direction, the small Anodonta 

 imbecillis Say, is hermaphrodite, that is: part of the gonad is 

 ovar}^ another is testis. Other Anodontae have not been suffi- 

 ciently examined in this respect. The balance of our Unionidae 

 are typically unisexual, yet among Quadrula, and even Lampsilis 

 (parva, Barnes), bisexual individuals are occasionally found. 



* Presented at the Akron meeting of the Ohio Academy of Science,. 

 November 25, 1910. 



