238 ■ MEMOIRS OF THE CARNEGIE MUSEUM. 



Genus Amygdalonaias Crosse & Fischer (1893). 

 Ortmann, 1912, p. 327; Simpson, 1914, p. 306 (as subgenus of Plagiola). 



Type Unio cognatus Lea. 



The nomenclature of this genus must remain provisional, until we know the 

 anatomical structure of the type-species. Two species have been reported from 

 Pennsylvania. 



Key to the Species of Amygdalonaias. 



ai. Shell short and high, larger A. truncata. 



02. Shell elongated and low, smaller A. donaciformis. 



Amygdalonaias truncata (Rafinesque) (1820). 

 Plagiola elegans (Lea) Simpson, 1914, p. 307; Amygdalonaias truncata (Rafin- 

 esque) Utterback, 1916, p. 148.^^° 



Plate XIV, fig. 7. 

 Records from Pennsylvania: 



Rhoads, 1899 (Ohio River, Coraopolis, Allegheny Co.). 

 Ortmann, 19096, p. 192. 



Characters of the shell: Shell rather small, moderately thick. Outline sub- 

 triangular-ovate, short and high. Anterior margin rounded, curving into the lower 

 margin, the latter nearly straight posteriorly, and often somewhat concave. Upper 

 margin short, curved, passing in a blunt angle or insensibly into the obliquely 

 descending posterior margin, which joins the lower margin in a more or less distinct 

 angle, so that the posterior end of the shell appears pointed. Beaks more or less 

 elevated, somewhat incurved, situated somewhat in front of the middle. Beak- 

 sculpture rudimentary, consisting of three or four fine bars, the first subconcentric, 

 those following double-looped; the posterior loop is triangularly pointed, and its 

 ascending part on the posterior slope is obsolete. Valves convex, more strongly 

 so in the anterior part of the shell, flattened upon the sides, and often with a faint 

 depression in front of the posterior ridge. The latter is distinct, often elevated 

 and keel-like towards the beaks. The posterior slope appears trmicate, slightly 

 convex, flat, or even slightl}^ concave. 



Epidermis yellowish brown or greenish, mostly with well-developed rays. 

 Rays wide or narrow, straight. Very often there is a pattern of dark spots upon 

 the rays, which may be arrow-shaped, or wavy, and may extend over a large part 



"° Vanatta (1915, p. 553) does not accept Rafinesque's specific name on account of Unio truncata 

 Spengler (1793); however, this does not conflict, since Rafinesque called his species Truncilla truncata. 

 See also Walker, 1916, p. 45. 



