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 |]. 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, 
l. ¢., 718, pp. 550 ff.) 
I have described the anatomy of the flat headwater-form 
(argenteum) under the incorrect name of P. fassinans. (See: 
Nautit. 28, ’14, p. 31.) Gravid females belonging to this have 
been collected on the following dates: May 11, ’13; May 12, 
715; May 13, 14, 15, 16, 20, 718; May 20, ’15; July 5, 7, 8, 9, 
10, 13, 14, 718. 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 Plewrobema-structure, much like that 
of P. clava (Ortmann, Ann. Carn. 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 4 
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 
