ORTMANN: monograph of the naiades of PENNSYLVANIA. 139 



pattern. The latter consists of concentric hands of lighter and darker color, 

 largely corresponding to growth-periods. Upon the lighter bands the rays are 

 sometimes visible. However, very often there are one to three more distinct, 

 dark green to black rays upon the posterior slope, which are best visible in trans- 

 mitted light. In many specimens, chiefly in the common creek-form, the epidermis 

 is uniformly blackish. 



Hinge edentulous, forming a practically straight line, with only the faintest 

 trace of a light undulation under the beaks. Beak-cavity shallow or moderate. 

 Dorsal muscle-scars faint, situated in the beak-cavity. Nacre white, as a rule, 

 silvery, bluish white, or cream-color, rarely pinkish, generally highly iridescent, 

 with blue, purple, and greenish reflections. In some cases it is more distinctly 

 reddish (salmon to rusty), but this is pathological (see below under salmonia). 



There is a difference in the shape of the shell of the two sexes, but this is very 

 slight, and not always present. In the females, and chiefly in the larger ones, the 

 shell is more swollen in the posterior middle portion of the disk, just in front of 

 the indistinct posterior ridge, in the region where the marsupium is located. This 

 swelling sometimes renders the lower margin more curved and slightly prominent 

 behind the middle. In extreme cases the shell is even slightly depressed in front 

 of the swelling. In young and half-grown shells this shape is not noticeable, and 

 even older shells do not always show it. Where it is distinctly developed, it can 

 be safely taken for an indication of the female sex, but not all females have this 

 character. It also should be said, that in the tj^pical (creek-) form, this sexual 

 difference is much less evident, than in the form of quiet water (gigantea). 



■ L. H. D. 



Size: 1. Linesville, Cat. No. 61.845 (largest on hand) .... 128 mm. 70 mm. 44 mm. 



2. Edinboro, Cat. No. 61.3648 ( 9 gravid) 126 



3. Waynesburg, Cat. No. 61.4729 ( 9 ) 123 



4. Cochranton, Cat. No. 61.3661 (9 gravid) 118 



5. Jamestown, Cat. No. 61.36.59 (d^) 105 



6. Waynesburg, Cat. No. 61.4729 (cf ) 91 



62 " 44 



68 " 40 



68 " 39 



61 " 41 



52 " 33 



Soft parts (See Ortmann, 1912, p. 292). Glochidia figured by Lea, Obs. VI, 

 1858, PL 5, figs. 32-34 and Surber, 1912, PL 3, fig. 45. Surber's measurements 

 are: 0.41 X 0.42, while I gave: 0.36 X 0.37 mm. 



Breeding season: Gravid females have been frequently found from August 6 

 to November 12, and again on May 22. This species seems to discharge very 

 early in spring (April). May 22 seems to be an exceptionaUy late date. 



Remarks: This is a very variable species, for which a great number of specific 

 names has been introduced, chiefly by Lea. The typical form (of which ovata 



