AND EMBRYONIC FORMS, 425 



one-third down the posterior edges. Mantle rather thick, thickened and colored on the 

 margin. Branchial opening very large, with numerous small colored papilla?. Anal 

 opening large, with numerous very small colored papillae. Super-anal opening rather 

 large, colored within and united below. Color of the mass dirty white, 

 Cincinnati, Ohio, J. Clark ; and Columbus, Ohio, H. Moores. 



Unio LiEVlSSlMUS, Lea, (Si/mphaiwta,) Trans. Amer. Phil. Soo. (2,) vol. iii. pi. 13, fig. 23, and Obs. vol. 

 i. p. 58. 



Branchial uterus occupies one-third of the posterior portion of the abdominal sack, 

 is pendent, curving posteriorly, and is crenulate on the inferior edges. Branchiae very 

 small, slightly rounded below, inner ones the larger anteriorly, but small posteriorly, 

 united the whole length of abdominal sack. Palpi rather large, rather transverse, 

 subelliptical, united half way down the posterior edges. Mantle thin, very much 

 thickened at the posterior inferior border, and extended into the wing. Branchial 

 opening small with very minute colored papillae. Anal opening rather large, without 

 papilla?. Super-anal opening large, united for some distance below. Color of the- mass 

 whitish. The adductor muscles are very large, and the anterior tractor muscle is so 

 large as to make a cicatrix like that of Iridina. The dorsal muscles are numerous 

 and large. 



Embryonic shell wedge-shape, clear white, with a hook -like process at each 

 lower angle. See Journ. A_cad. Nat. Sci. (2,) vol. iv. pi. 5, fig. 24. Obs. vol. vi. 

 p. 48. 



Cincinnati, Ohio, J. Clark ; and Columbus, Ohio, H. Moores. 



Remarks.— The wedge-shape form of this embryonic shell is very remarkable. I have 

 observed but two others which take this form, viz., Unio alatus and Unio purpu- 

 ratus. It will be observed that they form, in this character, a distinctive group, totally 

 different from all the other species yet observed. In the branchial uterus ova and well 

 formed embryonic shells were found. In the old specimens I found that the space 

 between the anal and super-anal openings was interrupted by one, two or three holes, 

 the edges being disunited. 



Unio tuberculatus, Bar. Am. Journ. Sci. vol. vi. p. 125. 



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



Branchice very wide, rounded below, inner ones much the larger, free two-thirds the 

 length of the abdominal sack. Palpi very large, thin, nearly transverse, subtriangular, 

 united nearly half way down the posterior edges. Mantle rather thin, with a broad 

 thickened border. Branchial opening rather large, with numerous, rather large, dark 

 papilla?. Anal opening enormously large, with numerous very minute papilla?. Super- 

 ior 



