74 NAIADES OF MISSOURI 



Plethobasus aesopus (Green).' 



("Bull Head," "Sheep's Nose," "Clear Profit.") 



PL XX, Figs. 56 A and B. 



1827 — Unio aesopus Green, Cont. Mac. Lyceum, I, p. 46, fig. 3. 

 1834 — Unio cypJiia 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. 



