THE NAUTILUS. 105 



dency whatever to unite the edges of the mantle at any point (see 

 pi. VI, fig. 1). The diaphragm is unique in this genus: the outer 

 lamina of the outer gills is posteriorly not connected with the mantle 

 for a considerable .distance, in consequence of which the posterior 

 end of the gills (diaphragm) is drawn into a projecting, free point, 

 forming a very incomplete diaphragm between the anal and branchial 

 opening. And finally the gills possess the peculiar feature that they 

 have no water tubes, that is to say the connection between the two 

 laminae is not formed by continuous septa, but by isolated and irreg- 

 ularly scattered subcylindrical projections of the tissue, which some- 

 times are arranged in indistinct and diagonal rows. 



Unio and Quadrula, on the contrary, show a tendency to more 

 sharply separate branchial and anal opening, and to develop a supra- 

 anal. The diaphragm is formed by the gills, which run backward 

 to or close to the mantle margin, the outer lamina} of the outer gills 

 being connected with the mantle to their ends. Thus the edges of 

 the mantle are drawn together at this point, but without growing 

 together, which effects a sharp separation of the anal and branchial 

 opening. Further, the anal is limited above by the coalescence of 

 the inner mantle edges. This junction is rather short, and above it 

 a part of the original anal is left open, the supraanal opening (the 

 genus Rotundaria makes an exception, and has an anal like Marga- 

 ritana ; also in the subfamily Lampsilince we have an exception in 

 Lampsilis parva (Barn.), where the anal is entirely closed above, at 

 least sometimes). The branchial is better defined in front in these 

 forms by the sudden disappearance of the papillce of the inner edge. 



Finally the structure of the gills is characteristic; here there are 

 well developed water tubes, formed by septa running from base to 

 edge of the gill, generally without any interruption. 



Unio and Quadrula are also characterized by the marsupium (the 

 marsupium of Margaritana is unknown). The water tubes (ovisacs) 

 are narrower, and the septa are stronger and closer together than in 

 the non-marsupial gills (see pi. VII, figs. 1 and 2). Either all four of 

 the gills (Quadrula, pi. VII, fig. 1) or only the outer ones(/wo, pi. 

 VII, fig. 2), in their whole length, are used as marsupium. There 

 are no additional differentiations in the marsupial gills, the gills 

 simply swelling during the period of gravidity, and the edges of the 

 gills do not undergo any change (this is different in the Anodontin& 

 and Lampsilina). 



