AND EMBRYONIC FORMS. 137 



ovarium. Branchiae large, nearly semicircular, inner ones much the larger, free 

 three-fourths the length of the abdominal sack. Palpi not very large, rather thick, 

 elliptical, united about one-third down the posterior edges. Mantle thin, with a broad 

 margin, attached near the centre of the cavity of the valves on each side posterior to 

 the anterior tractor cicatrix, and forming there a well impressed cicatrix. Branchial 

 opening rather large, with numerous small, brown papillae. Anal opening rather 

 small, with numerous, very minute, brown papillae. Super-anal opening large, united 

 below for a short distance. Color of the mass whitish. 



Columbus, Mississippi. Dr. Spillman. 



Remarks. — This well characterized species is in the possession of a ventral cicatrix, 

 formed by the adhesion of the middle of the mantle. In some specimens this mus- 

 cular attachment is large, in others it is small and sometimes obscure. 



Unio asper, Lea. Trans. Amer. Phil. Soc, (2), vol. iv. pi. 9, fig. 15. Obs. vol. i. p. 95. 



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



Branchiae large, nearly semicircular, inner ones much the larger, free two-thirds the 

 length of the abdominal sack. Palpi large, oblique, subelliptical, united half way 

 down the posterior edges. Mantle thin, having a broad thickened margin, blackish 

 on the posterior outer edges. Branchial opening rather large, with numerous small, 

 dark brown papillae. Anal opening small, blackish on the inner edges, without 

 papillae, but with a very slight disposition to crenulation. Super-anal opening very 

 large, colored on the edges and united below. Color of the mass whitish. 



Columbus, Mississippi, Dr. Spillman. 



Remarks. — In this species we have another Unio which has no papillae on the anal 

 opening. 



Unio atrocostatus, Lea. Trans. Amer. Phil. Soc, (2), vol. x. pi. 2, fig. 5. Obs. vol. iv. p. 44. 



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



Branchiae very large, very nearly semicircular, inner ones much the larger, free three- 

 fourths the length of the abdominal sack. Palpi rather large, oblique, subangular 

 posteriorly, united one-third down the posterior edges. Mantle thin, with a very 

 broad, thickened margin. Branchial opening small, with very small, brown papillae. 

 Anal opening large, with very minute colored papillae. Super-anal opening large, 

 slightly colored within and very slightly colored below. Color of the mass whitish. 



Columbus, Mississippi, Dr. Spillman. 



Unio declivis, Say. Trans. Jour. Med., vol., iv., 1831. 



Branchial uterus occupies the whole width of the outer branchiae. Branchiae very 

 wide, slightly curved below, inner ones much the larger, free nearlj' the whole 



111 



