UNIO. 493 
are more expressed and distinct than in the latter, more unequal and more numerous 
than in the former; the general shape is not so rounded as in U. nicklinianus and 
more compressed than in U. digitatus ; the posterior margin of the wings descends more 
obliquely, less abruptly than in either of these species. Kiister! gives ‘“‘ Mexico” as 
locality, probably because he identified it with the following species; his spelling 
“ mcklianus” is a mistake for “ nicklinianus,” as may be seen from the works cited. 
Unio boykinianus, Lea, Obs. Gen. Unio, iii. p. 46, t. 9. fig. 22, from Chattahoochee 
river, Georgia, is very like this species in sculpture, but it is much smaller and more 
swollen. 
3. Unio nicklinianus. 
Unio nicklinianus, Lea, Trans. Am. Phil. Soc. v. p. 28, t. 1 (1832)*; Obs. Gen. Unio, i. p. 140, t. 1°, 
and iii. p. 72, nota*; Hanley, Cat. Rec. Bivalv. p. 175, t. 21. fig. 52*; Sowerby, in Reeve’s 
Conch. Icon. xvi., Unio, t. 53. fig. 276°; Fisch. & Crosse, Miss. Scient. Mex., Moll. ii. p. 565°. 
Nearly circular, the posterior dorsal margin much ascending, posteriorly rapidly descending, the ventral margin 
strongly rounded ; the plaits broad, but flat, not very unequal, none on the ventral margin, 
a. Long. 148; alt. vert. 74, ale 121, diam. 52 millim. Vertices in + long. 
eS  . 144: ,, 88, 5, 116, ? 1 
” . 9 ” 7 
a, from Lea’s; b, from Sowerby’s figure. 
Hab. N.E. Mexico: Rio Moctezuma, an affluent of the southern Rio Panuco 
(Dr. Blanding *). 
II. Psoronaras, Fisch. & Crosse. 
4. Unio semigranosus. (Tab. XXX. figg. 1-4.) 
Unio semigranosus, Von dem Busch, in Philippi’s Abbild. neuer Conch. i. p. 19, t. 1. (Unio) figg. 1-3 
(1842)’; Hanley, Cat. Rec. Bivalv. p. 381, t. 20. fig. 33°; Kiister, in Martini & Chemnitz, 
Syst. Conch.-Cab. ed. 2, Unio, p. 252, t. 81. fig. 1°; Fisch. & Crosse, Miss. Scient. Mex., 
Moll. ii. p. 569‘; Pilsbry, Proc. Acad. Phil. 1892, p. 339°. 
Upper half of shell distinctly granulated. General outline oval-oblong; ventral margin feebly convex; height 
about 3—3 of the length of the shell; transverse diameter, height about 3 of the length. 
Hab. N.E. Mexico: Rio Panuco, at Tampico (V. d. Busch 1‘), 
S.E. Mexico: Rio ‘lulija in Tabasco (Rovirosa °). | 
Figg. 3 and 4 represent full-grown specimens from the late Prof. Dunker’s collection ; 
fig. 3 an example from Tampico. 
Var. testudineus. 
Unio testudineus, Morelet, Test. Noviss. i. p. 28 (1849)°; Fisch. & Crosse, Miss. Scient. Mex., 
Moll. ii. p. 571, t. 62. fig. 3, t. 70. fig. 37 (nec Sowerby). 
Unio semigranosus, Sowerby, in Reeve’s Conch. Icon. xvi., Unio, t. 10. fig. 36°. 
Sculpture as in U. semigranosus, but extending over nearly the whole surface ; general outline more rounded ; 
ventral margin very convex; height about 7, diameter about 2 of the length. 
Hab. N.W. Guatemata: Rio Usumacinta (Morelet °"). 
