AND EMBRYONIC FORMS. 439 



much thicker and it is not quite so much inflated. The ovisacks are extended below 

 the line of the branchiae, and in one of the specimens several of the ovisacks were 

 produced and extended below the margin. In another female I found some of the 

 ovisacks in the cavity below the anus, apparently passing out in that direction, while 

 the embryonic shell had not yet formed. 



Unio castaNus, Lea. Trans. Am. Phil. Soc. (2), vol. iv. pi. 11, Fig. 21. Obs. vol. i. p. 101. 



Branchial uterus . No ova were found here, but they were in the ovarium. 



Branchim very thin, very small, nearly straight below, rounded at the ends, united 

 the whole length of the abdominal sack. Palpi small, subtriangular, united one-third 

 down the posterior edges. Mantle very thin, with rather a broad margin and with 

 small distant papillae below the branchial opening. Branchial opening small, with 

 rather large, brownish papillae. Anal opening very small, with very minute papillae. 

 Super-anal opening very small, colored within and united below. Color of the mass 

 whitish. 



Columbus, Mississippi, Dr. Spillman. 



Unio Nashvillensis, Lea. Trans. Am. Phil. Soc. (2) vol. v. pi. 14, fig. 48. Obs. vol. i. p. 212. 



Branchial uterus occupies the posterior half of the outer branchiae, consisting of 

 twenty to thirty ovisacks in each leaf, extending beyond the line of the branchiae 

 and forming a semilunate lobe. Branchiai rather large, inner ones much the larger, 

 united the whole length of the abdominal sack. Palpi small, suboval, united only at 

 the upper part of the posterior edges. Mantle thin, with a broad margin, thickened 

 at the edges and furnished with papillae below the branchial opening, where there is a 

 dark line. Branchial opening rather large, with numerous, small, brownish papillae. 

 Anal opening very small, with numerous small papillae on the inner edges, where it 

 is nearly black. Super-anal opening rather small, colored on the inner edges and 

 united below. Color of the mass whitish. 



Embryonic shell elongate pouch-shape, light brown, has no hooks, very similar to 

 securis. See Journ. Acad. Nat. Sci. (2), vol. iv. pi. 5, fig. 6, and Obs. vol. vi. p. 47. 



Columbus, Mississippi, Dr. Spillman. 



Unio nux, Lea. Trans. Amer. Phil. Soc. (2), vol. x. pi. 24, fig. 43. Obs. vol. v. p. 39. 



Branchial uterus . No ova were found here, but they were in the ovarium. 



Branchiae large, thin, rounded below, the posterior ends not attached at the point, 

 but a short distance below, by a U shaped muscle, to the side of the branchial open- 

 ing, free nearly the whole length of the abdominal sack. Palpi rather large, sub- 

 oval, united for a short distance on the posterior edges. Mantle very thin, with 

 rather a broad margin. Branchial opening small, with very minute brown papillae. 



