ORTMANN: monograph of the naiades of PENNSYLVANIA. 173 



Alasmidonta (Prolasmidonta) heterodon (Lea) (1830). 

 Alasmidorda heterodon (Lea) Simpson, 1914, p. 499; Alasmidonta (Prolasmidonta) 

 heterodon (Lea) Ortmann, 1914, p. 44. 



Plate XII, figs. 1, 2. 

 Records from Pennsylvania: 



Lea, 1830 (Schuylkill River and Darby Creek, the latter cliicfly iu Delaware Co.) 



Conrad, 1838 (Schuylkill River). 



Gabb, 1861 (Sclniylkill River, below Fairmont Dam, Pliiladelphia.) 



Hartman & Michener, 1874 (Schuylkill River, Chester Co.) 



MarshaU, 1895 (Philadelphia). 



Schick, 1895 (Schuylkill Canal at Philadelphia and Manayunk; Neshaminy Creek, Bucks Co.) 



Ortmann, 19096, p. 207. 



Characters of the shell: SheU small and rather thin. Outline siibrhomboidal 

 or subtrapezoidal to subovate, and more or less elongate, rounded anteriorly, 

 subangular behind, with an upper posterior angle (which may be obsolete), and a 

 more distinct and produced lower posterior angle. Lower margin gently curved 

 (chiefly in the male), or straight and even slightly concave (chiefly in the female). 

 Beaks somewhat inflated, but not very prominent, anterior to the middle of the 

 shell, but not very near the anterior end. Beak-sculpture consisting of three or 

 four (rarely traces of a fifth) bars, the first two concentric and simple, the following 

 ones with a distinct angle upon the posterior ridge, in front of which is a more or 

 less distinct, shallow sinus. The bars are moderately heavy and blunt, and the 

 sinus never assumes the shape of a re-entering angle, sometimes it is barely indi- 

 cated. Thus the beak-sculpture cannot be called double-looped. Upon the 

 posterior slope there are one or two fine radiating lines, indicating as many addi- 

 tional bars, which are obliterated on the disk. 



Shell more or less swollen and inflated, chiefly in the region of the posterior 

 ridge, and in the female; the sides are rather flat. Posterior ridge quite distinct, 

 generally more so in the female than in the male, but also in the latter this ridge 

 is distinct, although rounded. In the female, it becomes almost angular. The 

 posterior slope is somewhat compressed in the male, while in the female it appears 

 sometimes almost truncated. 



Epidermis greenish olive to brownish. Sometimes obscure (rarely more di.s- 

 tinct), simple and straight rays are present, but in other cases, rays are invisible, 

 and the color of the whole surface is uniform. Often there are indistinct con- 

 centric bands of lighter and darker green. Growth-rests indistinct. 



Hinge complete, with pseudocardinals and laterals, but teeth rather feeble. 

 Pseudocardinals two in each valve, compressed and crenulated, often the anterior 



