THE NAUTILUS. 51 



Gills with well- developed septa, running parallel to the gill 

 filaments and forming water tubes. However, the septa are 

 not all continuous, but are often interrupted, chiefly so toward 

 the proximal (basal) part of the gill, and, toward the edge, 

 frequently shorter septa are intercalated. In the female, all 

 four gills have marsupial structure : the septa are much heavier 

 and more closely set than in the male ; this structure is most 

 evident in the central parts of the gills, while at the anterior 

 and posterior ends it resembles more that of the male. The 

 heavy septa in the middle of the gills of the female are frequently 

 perforated by subcircular holes, so that here the interruptions of 

 the septa assume a rather regular arrangement. 



When gravid, all four gills of the female are charged. The 

 gills, when fully charged, are only slightly swollen, with edges 

 remaining sharp, and often there are no ova at the ends of the 

 gills, chiefly the anterior end. The outer gills are charged first, 

 and thus there are some individuals in which the inner gills 

 have not yet received ova. The ova only incompletely stick 

 together in the shape of placentae, and easily fall apart. 



Glochidia moderately large, subovate or nearly subcircular : 

 they represent, in outline, a segment of a circle cut off by the 

 hinge line ; but the circle is not regular, being more narrowly 

 rounded in the middle of the ventral margin. There is no trace 

 of hooks. Valves of the glochidium rather strongly convex. 

 Length and height about equal, 0. 19 mm. 



None of the gravid females collected on March 31 had glo- 

 chidia, but such were present in some specimens collected in 

 June. This, and the additional facts that in some females the 

 gills were not yet fully charged in March, and that some were 

 discharged in June, demonstrate that the beginning of the breed- 

 ing season falls at the end of the month of March and that it 

 lasts at least till June : a rather unusual time in North-American 

 Nayades. 



Color of soft parts grayish or brownish white, without any 

 marked or characteristic tints. 



This species originally was described as an Anodonta, and 

 Simpson (1900 and 1914) placed it in his group of Homogenae, 

 which largely (with the exception of the last three genera) 



