ORTMANN: monograph of the naiades of PENNSYLVANIA. 153 



double-looped, but in the last (sixth and seventh) the sinus is often not sharp, but 

 represented merely by a gentle undulation. There is a good deal of variation, 

 chiefly in the number of bars, but also in the sharpness of the sinus, which may be 

 acute or more or less blunt. 



In other characters the shells of the two species are much alike, but as a rule 

 A. cataracta inclines more toward the elongated shape, although it also possesses 

 comparatively short and high forms. In the elongated pond-forms, another 

 difference is noted in the position of the posterior end, which is more elevated 

 above the base-line, so that the lower margin of the shell is more strongly convex, 

 its posterior part running more decidedly upwards. But this character is not 

 observed in the creek-forms. Also the posterior "wing" is generally more distinct 

 in A. cataracta, although often very low. 



On the average the color of A. cataracta is brighter green than in A. grandis. 

 It is light to dark green, rarely shading into brownish and blackish, often very 

 brilliant, with concentric lighter and darker bands, and the dark green rays are 

 also more distinct upon the disk; though there are specimens which do not show- 

 any rays. The broad rays of the posterior slope are often strongly developed 

 and broadened, so as to render the whole posterior slope blackish green. 



In this species as in its allies, the female sex is sometimes indicated by a swelling 

 of the valves posteriorly to the middle, but not all females exhibit this character. 



L. H. D. 



Size: 1. Flinton, Cat. No. 61.3637 (largest on hand) 128 mm. 72 mm. 45 mm. j 



2. do. " " 61.3636 125 " 65 " 44 " I pond-form 



3. Lockhaven, Cat. No. 61.4173 (9 gravid) 112 " 56 " 42 " j 



4. Tioga, Cat. No. 61.4171 (9 gravid) 85 " 46 " 27 " "] 



5. Avondale, Cat. No. 61.4167 86 " 42 " 31 " I creek-form 



6. York Haven, Cat. No. 61.4727 (cT) 84 " 47 " 28 " J 



7. Schuylkill, Philadelphia, Cat. No. 61.816 87 " 50 " .33 " Iryoni-type 



These measurements do not express the maximum size, and considerably 

 larger specimens are on record. 



Soft parts practically identical with A. grandis (See Ortmann, 1912, p. 293). 

 Glochidia figured by Lefevre & Curtis, 1910, p. 97, fig. C, and 1912, p. 146, fig. C. 



Breeding season: Conner says (1907 and 1909) that this species is gravid from 

 September to May, and that the interim falls in June, July, and August. I have 

 collected gravid females on July 23, 1908; August 19, 1909; Aug. 21, 1908; Aug. 

 24, 1909; Sept. 5, 1909; March 19, 1911; April 24, 1909. 



Of these dates, July 23 is surely an exceptional case (only one specimen was 

 gravid among a large number). The dates in August mark the beginning of the 



