126 MEMOIRS OF THE CARNEGIE MUSEUM. 



sharp. The later bars are only developed in the middle, and have only a slight 

 sinuation (see Marshall's figure, 1890, fig. 10). Shell rather flat, and more or less 

 compressed, with an indistinct, blunt posterior ridge. Greatest diameter in the 

 middle of the disk, or somewhat posterior to it, immediately in front of the posterior 

 ridge, and in this case the sides of the disk are flattened. Behind the posterior 

 ridge, the posterior slope is corrugated by heavy, blunt ridges or ribs, running 

 toward the posterior margin. In front of the posterior ridge, the disk is smooth, 

 without sculpture (sometimes there are short vertical grooves, such as are found 

 incidentally also in other Naiades) . 



Epidermis yellowish or brownish olive, greenish, brownish or blackish, with 

 more or less distinct, narrower or broader, dark green rays, which disappear in 

 old shells, which are more or less uniformly brownish or greenish black. Growth- 

 rests irregular, more or less distinct, often marked by darker bands. 



Hinge partly obliterated. The pseudocardinals are moderately developed 

 (sometimes heavy in old shells), of the characteristic Anodontine type (more or 

 less lamellar and directed obliquely forwards), with the interdental projection 

 very well developed. Lateral teeth practically absent, or indicated only by mere 

 rudiments. Beak-cavity shallow. Dorsal muscle-scars in the beak-cavity. Nacre 

 white, often cream-color or pale salmon, chiefly so toward the beak-cavities. 



No material sexual differences are seen in the shell. It has been asserted by 

 Scammon (1906) that the shell of the female is shorter and more inflated, while 

 Call (1900, p. 525) says that the females are a little more obese than the males. 

 According to my experience large specimens, which are rather swoUen just in 

 front of the posterior ridge, are generally females (See Nos. 1 and 3 in table of 

 measurements) but there are females, which do not show this character (See No. 2), 

 and in medium-sized and small shells it is not at all developed (Compare Nos. 

 4-9). As a rule, it is not easy to positively identify the sex by the shell alone, and 

 in younger shells this is impossible. That the females are shorter than the males 

 is not at all correct (See table of measurements). 



L. H. D. 



Size: 1. Enon Valley, Cat. No. G1.3134 149 mm. 86 mm. 50 mm. (swollen 9 , largest 



at hand) 



2. do. do. 137 " 76 " .37 " (flat 9) 



3. Waynesburg, Cat. No. 61.4709 134 " 77 " 45 " (swollen 9 ) 



4. Harbison, Cat. No. 61.4710 126 " 71 " 35 " (swollen cf ) 



5. Mount Morris, Cat. No. 61.4711 126 " 67 " 36 " ( 9 ) 



6. Waynesburg, Cat. No. 61.4709 122 " 72 " 35 " (d^) 



7. Harbor Bridge, Cat. No. 61.3709 122 " 67 " 32 " (flat 9, gravid!) 



8. Harbison, Cat. No. 61.4710 75 " 41 " 19 " (d') 



9. Wurtemberg, Cat. No. 61.4712 73 " 41 " 18 " (9) 



