44 MEMOIRS OF THE CARNEGIE MUSEUM. 



Beak-cavity not very deep. Dorsal muscle-scars on the hinge-plate. Nacre 

 white.^^ 



In this species, we meet with a more or less distinct sexual difference in the 

 shell, unusual in the subfamily Unionince. In the male the posterior margin of 

 the shell is short and subtruncate, and the ribs of the posterior slope are generally 

 well-developed and continued to the margin. In the female the posterior margin 

 is broadened and flattened in the region of the posterior angle, and the ribs of the 

 posterior slope are poorly developed and broadened, so that the posterior end of 

 the shell appears expanded. This expansion corresponds to the location of the 

 anal opening in the soft parts. 



L. H. D. 



Sise: 1. Edinburg, Cat. No. 61.3577 (sex doubtful) 156 mm. 86 mm. 43 mm. 



2. Mahoningtowu, Cat. No. 61.3574 (cf) 121 " 70 " 44 " 



3. Pulaski, Cat. No. 61.4548 ( 9 ) 130 " 64 " 37 " 



No. 1 is the largest specimen from Pennsylvania, but larger ones are known 

 from elsewhere. Scammon (1906) gives 168 as maximimi length. 



Soft parts (See Ortmann, 1912, p. 254). The glochidia have been figured by 

 Surber (1912, PL 2, fig. 31). They are unusually small, 0.085 X 0.100 mm. 



Breeding season: Sterki (19076, p. 48) collected gravid females on June 10, 

 1907, and Surber gives April to June. I did not find any gravid females in Penn- 

 sylvania, but collected some on May 9, 1913, in Pocatalico River, and on May 

 23, 1912, in West Fork River, West Virginia. All these had eggs only, filling all 

 four gills, forming lanceolate, not very solid, white placentae. Those from Poca- 

 talico River (two specimens) had the eggs brownish, discolored, and they were 

 being discharged when opened (about two hours after capture) . This was appa- 

 rently premature discharge, and the eggs were dead (suffocated) . 



Remarks: This species has a very characteristic outline and sculpture. The 

 latter, however, is quite variable. The tubercles of the disk may be larger or 

 smaller, regular or irregular in size, rounded, subtriangular, or even vertically 

 elongated. The ribs of the posterior slope are also variable, and more or less 

 distinct. Toward the lower margin, the sculpture often disappears, and in some 

 individuals the whole lower half of the shell may be smooth. The most remarkable 

 fact in this species is the sexual differentiation of the shell, which has led Simpson 

 to the opinion that it should represent a separate genus (Tritogonia) . However, 

 as I have shown, in all other characters, chiefly those of the soft parts, it is a true 

 Quadrula. The posterior dilatation of the shell in the female is in an entirely 

 different region from that in the females of the Lampsilince. 



''' In Pennsylvania, the nacre is always white; in the South (Alabama, etc.) it may be purple. 



