332 MEMOIRS OF THE CARNEGIE MUSEUM. 



than the rest of the surface. Concentric dark bands, marking growth-rests, are 

 more or less distinct. 



Hinge well-developed. Pseudocardinals two in left, one or two in right valve, 

 stumpy, ragged. Interdentimi absent. Laterals moderately long, moderately 

 strong. Beak-cavity rather shallow. Dorsal muscle-scars in beak-cavity. Ad- 

 ductor-scars distinct and well impressed, chiefly those in front. Nacre always 

 white. 



Sexual differences in the shell extremely great. The general form of the male 

 shell is ovate, somewhat pointed posteriorly, with a rather distinct posterior ridge 

 and a radial furrow in front of it. The female shell is also ovate, but the wider 

 part is behind, and it is not at all pointed at the posterior end, but broadly rounded. 

 The place of the radial furrow is filled by a broad postbasal expansion, which may 

 occupy the whole posterior half of the shell, and from the anterior part, this is 

 marked off, in old specimens, by a more or less distinct constriction. In young 

 females (PI. XXI, fig. 6), this postbasal expansion is less developed, and the shell 

 is only slightly thinner here than the rest of it. But in old females this expansion 

 is greatly developed, and is very thin, since the deposition of nacreous matter 

 ceases here, and thus the expansion appears chiefly horny (epidermis), with a thin 

 film of prismatic matter. The females grow practically to the same size as the 

 males. 



L. H. D. 



Size: (Males) 1. Meadville, Cat. No. 61.3362 72 mm. 50 mm. 31 mm. 



2. Cochranton, Cat. No. 61.3363 : .70 



3. do. " " do 61 



4. do. " " do 45 



5. Utica, Cat. No. 61.3368 37 



(Females) 6. Warren, Cat. No. 61.3979 70 



7. Cochranton, Cat. No. 61.3363 (gravid) 65 



S. Hickory, Cat. No. 61.3366 (gravid) 62 



9. Cochranton, Cat. No. 61.3363 51 



10. Harbor Bridge, Cat. No. 61.3372 36 



Soft parts (See Ortmann, 1912, p. 358, fig. 28). Glochidia (See ibid.) similar 

 to those of T. perplexa Lea (Obs. VI, 1858, PL 5, fig. 21) (= torwZosa Rafinesque). 



Breeding season: Gravid females were collected on Sept. 2, 1908; Sept. 5, 

 1908; Sept. 6, 1908; Sept. 15, 1909; Sept. 18, 1908. On the first date young 

 glochidia were already observed, and thus the beginning of the season probably 

 falls early in August. The species probably is hradytictic. 



Remarks: T. rangiana has been considered a variety of T. tdrulosa (Rafin- 

 esque) = perplexa Lea. The latter is not found in Pennsylvania. It is dis- 



