606 PROCEEDINGS OF THE NATIONAL MUSEUM. voL.xxn. 



fPLAGIOLA COGNATA Lea. 



* Unio cognatas Lea, Pr. Ac. N. Sci. Phila., IV, 1860, p. 306 ; * Jl. Ac. N. Sci. Phila., 



IV, 1860, p. 368, pi. Lxv, fig. 193; * Obs., VIII, 1860, p. 50, pi. Lxv, fig. 193.— 

 ^SowERBY, Conch. Icon., XVI, 1866, pi. xxxix, fig. 214.— *E. H.Wright, 

 Check List, 1888.—* Fischer and Crosse, Miss. Sci., Pt. 7, II, 1894, p. 602. 

 *Margaron ( Unio) cognatus Lea, Syn., 1870, p. 43. 



Eio Salado, New Leon, Mexico. 



Subgenus PTYCHODERMA Simpson, 1900. 

 (Type, Unio cyrenoides Philippi. ) 



Shell triangularly ovate or rounded, solid, bluntly angled before, 

 more sharply angular behind, with a tolerably well-marked posterior 

 ridge; surface strongly and irregularly concentrically sulcate; beaks 

 small but rather prominent, sculptured with fine, irregular, broken 

 ridges, which are somewhat doubly looped, the front loop being larger 

 and more rounded; epidermis olive to tawny, wrinkled, sometimes 

 having faint rays; pseudocardinals compressed, ragged; hinge plate 

 narrow; laterals short, slightly curved, obliquely striated; muscle 

 scars rather shallow, the posterior round; female shell produced in 

 post-basal region. 



Animal with small branchiae, inner united the whole length to the ab- 

 dominal sac, wider than the outer in front, narrower behind; marsupium 

 occupying the posterior part of the outer gills in from eleven to twenty 

 distinct ovisacs which are rounded and dark below, the whole having 

 a decided, parallel sulcus inside and out near its base; mantle thin, 

 papery, with a wide, thickened, double edge, which is sometimes cre- 

 nate; branchial opening large fringed; anal opening smooth or with 

 only the faintest crenulations.^ 



(Group of Plagiola cyrenoides.) 



Shell generally more or less covered with radiating grooves which cut 

 up the concentric sculpture into looped wrinkles or nodules, sometimes 

 having the appearance of dried, wrinkled paint, the sculpture extend- 

 ing into the substance of the shell; epidermic usually tawny or yel- 

 lowish, rarely showing any vestiges of rays; female shell greatly pro- 

 duced in the post-basal region. 



t PLAGIOLA CYRENOIDES Philippi. 



* Unio cyrenoides Philippi, Zeits. fiir Mai., IV, 1847, p. 93; *Abbild., Ill, 1848, p. 



49, pi. V, fig. 1.—* Conrad, Pr. Ac. N. Sci. Phila., VI, 1853, p. 248.— *H. and 

 A. Adams, Gen. Rec. Moll., II, 1857, p. 495.—* Kuster. Conch. Cab. Unio., 

 1862, p. 285, pi. xcvi, fig. 1.— *B. H. Wright, Check List, 1888.— * P^tel, 

 Conch. Sam., Ill, 1890, p. 150. 



* Margaron ( Unio) cyrenoides Lea, Syn., 1852, p. 25; 1870, p. 38. 



' In a female P. cyrenoides, which was not gravid, the sulcus near the base of the 

 inside of the marsupium was quite deep, in fact the part above it hung over it in a 

 sort of flap, which would no doubt disappear if it was filled with ova. 



