198 MEMOIRS OF THE CARNEGIE MUSEUM. 



Lower margin convex or straight, sometimes slightly concave in the middle. Upper 

 margin gently convex, forming a more or less distinct angle with the obliquely 

 descending posterior margin. Beaks little prominent, situated in front of the 

 middle. Beak-sculpture distinct, moderatelj' heavy, consisting of about four 

 concentric bars, placed obliquely with reference to the hinge line, roimded anter- 

 iorly, angular upon the posterior ridge. A few radiating lines anteriorly and 

 posteriorly to the bars (See Marshall, 1890, fig. 12). 



Shell generally gently convex or rather flat upon the sides, but occasionally 

 more or less swollen or inflated. Posterior ridge indistinct and broadly rounded. 

 Posterior slope somewhat compressed, and, when young, often elevated at the 

 upper posterior angle. 



Epidermis j'ellowish, greenish, brownish, or blackish, with more or less distinct 

 gi'eenish or blackish raj^s. Rays straight, narrow or wider, often quite obsolete. 

 Posterior slope sometimes darker than the rest of the shell. Very old shells are 

 mostly quite black all over. Concentric darker bands may be present. 



Hinge much reduced. Lateral teeth entirely absent; pseudocardinals indi- 

 cated only by slight swellings of the hinge-line; rarely can the single elements be 

 made out. Generally, the hinge-line is only somewhat wavy under the beaks. 

 Beak-cavity moderate. Dorsal muscle-scars in the beak-cavity. Anterior ad- 

 ductor-scars rather distinct, posterior ones less so. Nacre bluish white, toward the 

 thick part of the shell pure white, or cream-color, and very often salmon-pinkish. 



Male and female shells practically indistinguishable. 



L. H. D. 



Size: 1. Cusli Cushion Creek, Cat. No. 61.3627 (cf ) 106 mm. 56 mm. 43 mm. 



2. Grove City, Cat. No. 61.3278 (9 gravid) 102 " 55 " 39 " 



3. Tioga, Cat. No. 61.4154 ( 9 gravid) 81 " 41 " 32 " 



4. Milesburg, Cat. No. 61.4158 (9 gravid) 74 " 42 " 27 " 



5. Warren, Cat. No. 61.4164 (cf ) 66 " 37 " 22 " 



6. Darlington, Cat. No. 61.2945 (c?) 44 " 25 " 16 " 



Soft parts: A poor figure was published by Lea (Obs. II, 1838, PI. 15, fig. 47) ; 

 a description was given by Ortmann, 1912, p. 299. Glochidia figured by Lea 

 (Obs. VI, 1858, PL 5, fig. 5), and Surber (1912, PL 1, fig. 3). Surber's measure- 

 ments are: 0.350 X 0.285, while mine {I. c, p. 300) are: 0.36 X 0.30 mm. 



Breeding season: Records for gravid females are most complete. They cover 

 the time from July 11 to November 4, and from April 17 to Maj' 22. It is a typi- 

 cally hradytictic form, with the interim between the breeding seasons falling, 

 in our region, in the end of May, June, and the beginning of July. Surber (1912) 

 gives a shorter interval, only in July. 



