500 MOLLUSCA. 
IV. Prievropema, Raf. 
18. Unio spheniopsis. 
Unio spheniopsis, Morelet, Test. Noviss. i. p. 29 (1849) *; Fisch. & Crosse, Miss. Scient. Mex., Moll. 
ii. p. 583, t. 61. figg. 2, 2a, b°. 
Trigonal, swollen, with a conspicuous ridge descending from the summits towards the hinder extremity, 
separating a distinct posterior area from the rest of the surface. 
Hab. N. GuateMaLaA: upper region of the Rio Usumacinta, Province of Peten 
(Morelet 1*). 
19. Unio couchianus. 
Unio couchianus, Lea, Proc. Acad. Phil. 1860, p. 305°; Journ. Acad. Phil. (2) iv. p. 371, t. 66. 
fig. 196°; Obs. Gen. Unio, viii. p. 53, t. 16. fig. 196 *. 
Near the preceding, but more quadrangular ; the ridge broader and somewhat more advanced, limited on each 
side by depressions ; some small plaits on the posterior area. 
Hab. N.E. Mexico: Rio Salado, in Nuevo Leon (Berlandier !-%), 
Small, short, oblique plaits on the posterior area are distinctly represented in the 
figure, but not mentioned in the description. 
20. Unio usumacintez. 
Unio usumasinte, Crosse & Fisch. Journ. de Conch, xl. p. 194 (1892)*; Miss. Scient. Mex., Moll. 
ll. p. 574, t. 63. fig. 4, t. 67. fig. 5%. 
Triangular, very swollen, with prominent summits and a descending keel behind them; sculpture irregular, 
consisting of plaits and tubercles. 
flab. N. GuateMALa: Rio Usumacinta (Morelet} 2). 
This species is intermediate between Psoronaias and Pleurobema, exhibiting the 
general form of the latter and the sculpture of the former. In the sculpture and 
triangular form it approaches also the group of U. gabbianus (Arotonaias), but differs 
from them, as well as from Psoronaias, in the very swollen summits. 
V. SpuEnonalss, Fisch. & Crosse. 
21. Unio licebmanni, (Tab. XXXIV. figg. 1, 1a, 6.) 
Unio liebmanni, Philippi, Zeitschr. f. Malak. iv. p. 96 (1847) ’; Abbild. neuer Conch. iii. p. 87; t. 6. 
fig. 1°; Kiister, in Martini & Chemnitz, Syst. Conch.-Cab. ed. 2, Unio, p. 281, t. 94. fig. 7°; 
Fisch. & Crosse, Miss. Scient. Mex., Moll. ii. p. 616+. 
Solid, heavy, convex, rounded and somewhat produced before, distinctly beaked behind; ventral margin a 
little sinuated ; summits broad, not prominent; inside pale purplish. 
a. Long. 102; alt. vert. 50, ale 44, diam. 36 millim. Vertices in 2 long. 
b. ” 101; oo) 50, ” 45, ” 40 ” ” 3 or 
C. ” 93; ” 443, » 41, 4, 33 ” ” 4 ” 
a, Figure given by Kuster; 6. Philippi’s original figure; c. Specimen given by the Copenhagen Museum to 
that of Berlin. 
Hab. Mexico, without nearer indication of locality (Liebmann * 1), 
* Prof. F. Liebmann, of Copenhagen, travelled in Mexico, in 1840-18438, not only on the eastern slope 
(Vera Cruz, Papantla, &c.), but on the western also (Acapulco, &c.). 
