MOLLUSCA. 649 
Quadrula guatemalensis, of the same author (loc. cit. p. 83, t. 2. fig. 4), from the Rio 
Usumacinta, is founded on a young specimen of this or of a nearly allied species. 
AROTONAIAS (pp. 480, 497). 
This subgenus, comprising chiefly Nicaraguan species, coincides with Ptychoderma, 
section Plagiola, in Simpson’s arrangement (loc. cit. p. 606). The sheet of the 
present work (60) in which Arotonaias was characterized (anted, p. 480) was published 
in February 1900, and my name almost certainly has priority. For U. aratus and 
U. granadensis, however, he proposes a distinct section, Micronaias, within the genus 
Unio (loc. cit. pp. 743, 744), and U. caldwelli and U. averyt are placed by him in the 
genus Nephronaias (loc. cit. p. 593). 
17(a). Unio reticulatus. 
Nephronaias reticulata, Simpson, Proc. Acad. Phil. 1900, p. 77, t. 2. fig. 3°. 
Elliptical, slightly triangulate behind, with radiating ridges or slight furrows, which cut the concentric 
suleations into loops, and give the shell a reticulated appearance; outside tawny. 
Long. 50, alt. 33, diam. 20 millim. Summits in 2 of the length. 
Hab. Honpuras: Patook River !. 
Simpson (/oc. cit. pp. 594, 595) makes this species the type of a separate group 
of the genus Mephronaias, and places it between that of NW. macnieli and that of 
NV. scamnata ; these latter were included by me in Simonaias. 
PLEUROBEMA (pp. 480, 500). 
The three species included under this subgeneric name are placed by Simpson in 
the genus Quadrula, division Tetragene, and not in that of Plewrobema, Raf., type 
U. clava, Lam., division Homogene. 
SPHENONAIAS (pp. 480, 500); NepHRonatas (pp. 480, 502) ; 
, Simonatas (pp. 480, 505). 
The species enumerated under these three subgenera are distributed by Simpson in 
quite a different manner between two genera—Unio, in the restricted sense, division 
Homogene, and Nephronaias, division Heterogene. U. aztecorum is even placed in 
Lampsilis, apart from U. plicatulus, Charp., Kiist. (Mephronaias), with which I believe 
it is identical. 
Unio acutirostris (p. 501). : 
On account of the vague indication of locality “South America,” U. acutirostris, 
Lea, is associated by Simpson (doc. cit. p. 885) with other South-American species in 
the genus Diplodon, far apart from U. sphenorhynchus, Crosse & Fisch., with which 
I suspect it to be identical. Itis true that I have not seen a specimen determined 
by Lea. 
ee CENTR.-AMER., Terr. and Fluviat. Mollusca, Aprii 1901. 82 
