THE NAIADES OF MISSOURI 185 
° Plethobasus aesopus (Green).! 
(“Bull Head,” ‘“‘Sheep’s Nose,’’ ‘Clear Profit.’’) 
PIX. Figs: 56°Ay and-B: 
1827—Unio aesopus Green, Cont. Mac. Lyceum, I, p. 46, fig. 3. 
1834— Unio cyphia Conrad, New F. W. Shells, p. 68. 
1900b—Pleurobema aesopus Simpson, Proc. U. S. Nat. Mus., p. 764. 
- 1912b—Plethobasus aesopus (Green) Ortmann, An. Car. Mus., VIII, 
pp. 260-261. 
ANIMAL CHARACTERS. 
NUTRITIVE STRUCTURES:—Branchial opening densely set 
with long and short papillae; anal smooth; supra-anal connected 
to anal by short mantle attachment; gills long and sharply pointed 
posteriorly, inner gills wider in front, inner laminae free from 
visceral mass; palpi short, wide; color of soft parts, peculiar, 
mostly orange, the foot, adductors and mantle margins being a 
brighter orange. 
REPRODUCTIVE STRUCTURES:—Only outer gills marsupial; 
- ovisacs, when gravid, swell moderately in the center leaving their 
unswollen distal ends pointed, thus giving their conglutinates 
narrow, lanceolate shape which are solid, red and discharged in 
unbroken form; glochidia semicircular, ventral margin obliquely 
rounded, hinge line long, medium in size, length slightly greater 
than height (0.220 x 0.200mm). 
SHELL CHARACTERS. 
EXTERNAL STRUCTURES:—Shell triangular, rounder before, 
pointed behind, rather heavy; post-umbonal ridge rounded; 
umbones high and full, tilted anteriorly, incurved, sculptured 
by a deep furrow just posterior to tip of each beak and by a few 
coarse concentric ridges and fine radiating lines; disk sculptured 
by a row of six or eight, coarse, elongated, undulated or hummocky 
tubercles extending from beaks to ventral margin with a broad, 
shallow trough between this tubercled row and the post-ridge; 
epidermis dark straw to glistening yellowish brown. 
INTERNAL STRUCTURES:—Cardinals double in left, single in 
right; interdentum narrow; laterals double in left only faintly 
‘ According to Rafinesque’s evident description of Green’s aesopus 
in his Monograph of 1820 (p. 39) under the name of Obliquaria cyphya 
(U. cyphia) this species should really bear the name now of Plethobasus 
cyphius (Raf.) because of priority. 
