THE NAUTILUS. 85 



becoming indistinct and effaced in the middle part. Posteriorly, 

 upon the posterior ridge, there are a few (two to three) low, 

 indistinct tubercles, which show no connection with the anterior 

 bars. Thus, anteriorly, the beak-sculpture resembles that of 

 L. subplana, but posteriorly it is different in the development 

 of low tubercles. 



The description of the shell, as given 1. c. for the genus, 

 should be slightly modified so as to include this species. This 

 refers chiefly to general shape and color pattern of the shell 

 (rays breaking up into blotches). 



PLEUROBEMA OVIFORME (CONRAD), P. OVIFORME ARGENTEUM 

 (LEA), P. OVIFORME HOLSTONENSE (LEA). (See: Ortmann, 

 1. c., '18, pp. 550 ff.) 



I have described the anatomy of the flat headwater-form 

 (argenteum^ under the incorrect name of P. fassina f iis. (See: 

 NAUTIL. 28, '14, p. 31.) Gravid females belonging to this have 

 been collected on the following dates: May 11, '13; May 12, 

 '15; May 13, 11, 15, 16, 20, '13; May 20, '15; July 5, 7, 8, 9, 

 10, 13, 14, '13. Glochidia have been secured on May 11, 15, 

 20, and July 8 and 9. This is a tachytictic form, breeding 

 from May to July. 



Soft parts of the typical Pleurobema-structure, much like that 

 of P. clava (Ortmann, Ann. Cam. Mus. 8, '18, p. 234). Mantle- 

 connection between anal and supraanal openings short, some- 

 times absent. Anal with very fine papillae, branchial with 

 larger papillae. Posterior margins of palpi connected for ^ to 

 of their length. Inner lamina of inner gills free from abdom- 

 inal sac, except at anterior end. In the female, the outer gills 

 alone are marsupial. When charged, the placentae are rather 

 distinct, but less so when glochidia are present. They always 

 are lanceolate and compressed, never subcylindrical. Glochidia 

 subelliptical, almost subcircular, about as high as long, L. and 

 H. 0.16 mm. (much like those of P. clava). Sometimes they 

 are slightly higher than long, L. 0.15, H. 0.17 mm. (So in 

 specimens from Chickamauga Creek, Ringgold, Ga. ) 



Color of soft parts whitish, often with the foot yellowish, pale 

 brown, or pale orange, rarely also mantle-margin and adductors 



