April, 1911.] Anatomy and Physiology of the Unionidae. 
33 1 
20. Orton, E. The coal fields of Ohio. Geol. Surv. Ohio 7: 255- 
290, 1893. See also subject index of Ohio reports in Bibli- 
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, 1S96. 
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., 18S9. 
25. White, D. The upper Palaeozoic floras, their succession and 
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NOTES ON THE ANATOMY AND PHYSIOLOGY OF THE 
UNIONIDAE.* 
V. Sterki. 
The Unionidae are not only the most conspicuous part of our 
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 
M argaritana, and those without hinge teeth were Anodonta. Con- 
ehologists 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 mollusca, have three very 
different functions: respiration, nutrition (as food gatherers), and 
as brood chambers for the ova and embryos. 
The general morphology and anatomy of the fresh-water 
mussels is well known, but the special features are frequently not 
mentioned, or very fragmentarilv, 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 
ovary, 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 2.5, 1910. 
