174 MEMOIRS OF THE CARNEGIE MUSEUM. 



in the right valve, and sometimes the posterior in the left valve rudimentary. 

 In addition, there is an interdental tooth in the left valve, which may be well- 

 developed and isolated, or may be more or less connected with the posterior pseudo- 

 cardinal, or may be even rudimentary. There is great variation in this respect. 

 Laterals rather thin and short; characteristically there are two laterals in the 

 right, and one in the left valve (just the reverse of the normal condition in the 

 Naiades), but there is considerable variation in the laterals. The upper one is 

 generally not so sharp as the lower one, and the former may be more or less rudi- 

 mentary, sometimes only for a part of its length. Beak-cavities moderately 

 developed, not deep. Dorsal muscle-scars distinct, in the beak-cavity. Adductor 

 muscle-scars moderatelj^ deep anteriorly, faint posteriorly. 



Nacre bluish or silvery white, iridescent, and often inclining toward cream- 

 color or yellowish in the beak-cavity; sometimes with greenish or grayish dis- 

 coloration. 



Shells of males and females distinguishable. The male as a rule has a more 

 compressed, and more ovate and elongate shell, with the lower margin gently and 

 uniformly curved, the posterior ridge less sharp, and the posterior slope not trun- 

 cate; while in the female the shell is more swollen in the region of the posterior 

 ridge and just in front of it; has a more distinct posterior ridge, which renders 

 the posterior slope more truncate, and makes the whole outline of the shell shorter 

 and more trapezoidal. Furthermore in the female the lower posterior angle is 

 more produced downward, so that the lower margin of the shell becomes rather 

 straight, in some cases even concave. In young specimens the sexual differences 

 of the shell are not seen, and prevalently the j^oung females in shape resemble males. 



L. H. D. 



Size: 1. Kunkletown, Cat. No. 61.4652 (cf ) 48 mm. 27 mm. 19 mm. 



2. Manayunk, Cat. No. 61.4250 (cf) 42 " 23 " 14 " 



3. do. Cat. No. 61.4249 (9 gravid) 40 " 23 " 16 " 



4. do. Cat. No. 61.4250 (cf ) 38 " 21 " 14 " 



5. do. " " do. (9 gravid) 38 " 21 " 15 " 



0. do. Cat. No. 61.4249 (9 gravid) 35 " 20 " 14 " 



*S'o/^ parts (See Ortmami, 1912, p. 295). Glochidia (See Ortmann, 19116, PI. 

 89, fig. 8). 



Breeding season: Conner (1909, p. 112) found this species gravid in February, 

 and I found gravid females with glochidia on April 24, 1909. 



Remarks: A small species, easily overlooked, with no striking external features. 

 However, the general shape, chiefly the distinct posterior ridge, give it the un- 



