OF THE GENUS UNIO. 271 



In all the other species which I have examined I have found 

 the ovaries and oviducts as described by Cuvier, Bosc, Ca- 

 rus, &c. The oviducts in these lie in a direct line between 

 the two great muscles, and are attached to the upper pair of 

 branchiae. In the irroratus this space is so small, as is also 

 the cavity, that it seems to require a different conformation to 

 accommodate the oviducts, and thus we find them pendent, 

 and not placed along the plane of the branchitE. The long 

 sacks containing the ova are inserted about half way up the 

 branchiai and somewhat posterior to the centre. The num- 

 ber of these sacks in my three specimens consists of eight in 

 two, and seven in the other. The posterior sack is the outer 

 or surrounding one, and measured two inches ; the second and 

 fourth 2-2; the third 2-4; the fifth 1-9; the sixth 1-6; the 

 seventh 1*4. In diameter the sacks are nearly the same size, 

 the interior ones being rather smaller than the exterior, which 

 measures one-twentieth of an inch. 



These measurements were effected by separating the mem- 

 branes which connect the sacks together and stretching them 

 out. The diameter of the cone is -6 ; its elevation -2 of an 

 inch. The outer sack terminates after making one revolu- 

 tion ; the second advances one-third on the succeeding revo- 

 lution, and each succeeding one obeys the same law until the 

 last terminates in the centre, and the mass having performed 

 three revolutions, the whole forms a depressed cone. 



This curious arrangement of the sacks to form the depres- 

 sed cone, which has its base resting on the region of the sto- 

 mach, is admirably calculated by the economy of nature to 

 harmonize with the construction of shell, which presents 

 only at the centre of its disks room for the essential purpose of 

 propagation. See plate V. 



Fig. 6 represents the interior, fig. 7 the exterior of the ovi- 

 duct, the mantle being removed. 



a the mouth. 



b the great anterior muscle. 



c the superior right branchiae. 



