794 PROCEEDINGS OF THE NATIONAL MUSEUM. 



Section PACHYNAIAS Crosse and Fischer, 1893. 

 (Type, Unio spheniopsis Morelet.) 



Shell elongate-triangular, inflated, truncated above and rounded 

 below in front, straight on the base, with a high, sharp posterior ridge; 

 the post base slightly biangulate; beaks full, the sculpture consisting of 

 apparently faint ridges which run parallel with the growth lines; 

 whole surface distinctly concentrically ridged ; epidermis without rays ; 

 pseudocardinals solid, stumpy, somewhat radiate, roughened; beak 

 cavities rather deep; muscle scars deep, smooth; nacre bluish white, 

 thicker in front. 



Animal unknown. 



tQUADRULA SPHENIOPSIS Morelet. 



* Unlo spheniopsis Morelet, Test. Nov., I, 1849, p. 29.— * Fischer and Crosse, 



Miss. Sci., Pt. 7, II, 1894, p. .583, pi. lxi,. figs. 2, 2a, 2b. 



Eio TJsumacinta, Guatemala. 



tQUADRULA RUGOSOSULCATA Lea. 



* Unio rugososulcatus IjEA, Pr. Ac. N. Sci. Phila., X, 1866, p. 33; *.J1. Ac. N. Sci. 



Phila.,VI, 1868, p. 266, pi. xxxiv,fig.81; ^Obs., XII, 1869, p. 26, pi. xxxiv, 

 fig. 81.— '^B. H. Wright, Check List, 1888. 



* Margaron ( Unio) rugososulcatus Lea, Syn., 1870, p. 35. 



Central America. 



Subgenus EOTUNDAEIA (Rafinesque, 1823) Simpson. 

 (Type, Ohliqiiaria tuherculata Rafinesque.) 



Shell rounded or quadrate, slightly truncate above in front; posterior 

 ridge low; beaks prominent, curved inward and forward over a strongly 

 marked lunule; beak sculpture consisting of numerous (20 to 30) fine, 

 irregular, broken, somewhat concentric corrugations which extend 

 well on to the second year's growth and gradually blend with the regu- 

 lar sculpture; posterior three-fifths of the shell tuberculate; epidermis 

 brown ; beak cavities very deep, compressed ; nacre violet. 



Animal having the gills very large, inner very much wider in front, free 

 the whole length from the abdominal sac; branchial opening immense, 

 with many small papillae; anal opening very large, smooth; superanal 

 opening not at all closed below.^ 



(Group of Quadrula tuherculata.) 



Shell rounded, truncated behind, with a wide depression down the 

 post slope. 



' I regret that I have never been able to see gravid specimens of either of the species 

 placed here. They are generally placed with the pustulusus group, but the strongly 

 developed sulcus on the posterior slope and the very remarkable beak sculpture, 

 which is not at all like that of .any of our species, are good distinguishing characters. 

 The enormous anal and branchial openings, and the open superanal opening are 

 characters of some value. 



