ORTMANN: MONQGRAPH of the naiades of PENNSYLVANIA. 181 



The exact southern boundarj^ of this species in North CaroUna is unknown. 

 Although its metropohs seems to be in the northern parts of the Atlantic watershed, 

 we have reason to believe, on account of its southern affinities, that it belongs to 

 the southern element in the Atlantic fauna (Ortmann, 1913a, pp. 324 and 361). 



Subgenus Decurambis Rafinesque (1831). 

 Rngifera Simpson, 1900, p. 670; 1914, p. 504; Ortmann, 1914, p. 46; Decurambis 

 Frierson, 1914a, p. 7. 



Type Alasmodonta niarginata Say. 

 Two species and one variety belong here. 



Key to the Forms of Decurambis. 

 fli. Posterior slope strongly truncate. Beaks placed more centrallj', anterior end of shell more produced. 

 6i. Shell greenish, posterior slope slightly lighter than the rest, but generally with rays. 



A. (D.) marginata. 

 62. Shell with posterior slope contrasted with the rest, light or reddish brown, hardly with rays. 

 Rest of the shell brownish or yellowish, with distinct green rays. 



A. (D.) marginata susquehannm. 



Oj. Posterior slope less strongly truncate. Beaks placed more anteriorly, and anterior end of shell 



shorter A. (D.) varicosa. 



Alasmidonta (Decurambis) marginata (Say) (1819). 

 A. truncata (B. H. Wright) Simpson, 1900, p. 671; .4. marginata (Say) Fox, 

 1901, p. 47; Alasmidonta marginata (Say) Simpson, 1914, p. 504. 



Plate XII, fig. 3. 

 Records from Pennsylvania: 



Harn, 1891 (western Pennsylvania). 



Rhoads, 1899 (Ohio River, Coraopolis, Allegheny Co.; Beaver River, Wampum, La\NTence Co.) 



Ortmann, 19096, p. 196. 



Characters of the shell: Shell of medium size, moderately thick, but rather thin 

 when young. Outline subtrapezoidal, rounded before and triangular behind. 

 Lower margin gently convex, more or less straight, or even somewhat concave in 

 the posterior part. Upper margin straight or gently convex, forming a more or 

 less distinct angle with the obliquely descending posterior margin. Beaks large 

 and inflated, somewhat elevated above the hinge-line, situated somewhat in front 

 of the middle, but at a considerable (although variable) distance from the anterior 

 end. Beak-sculpture heayj^, consisting of three or four thick bars, the second and 

 third being a little produced and angular upon the posterior ridge, and having a 

 more or less distinct sinus in front of the angle. This sinus is quite variable, some- 



