§20 MOLLUSCA. 
According to Stearns ®, who has examined the original specimen, U. manubius is one 
of the varieties of the North-American U. camptodon, Say. 
55. Unio explicatus. 
Unio explicatus, Morelet, Test. Noviss. i. p. 28 (1841) 1, Fisch. & Crosse, Miss. Scient. Mex., Moll. 
ii. p. 594, t. 61. fig. 1°. 
Unio testudineus, Sowerby, in Reeve’s Conch. Icon. xvi., Unio, t. 22. fig. 101° (nec Morelet). 
Hab. S.E. Mexico: Rio Usumacinta, near Balancan, State of Tabasco (Morelet ! ). 
XI. Metaptera, Raf. 
56. Unio paludosus. 
? Unio largillierti, Philippi, Zeitschr. f. Malak. iv. p. 94 (1847) '. 
Unio paludosus, Morelet, Test. Noviss. i. p. 30 (1849)?; Fisch. & Crosse, Miss. Scient. Mex., 
Moll. ii. p. 559, t. 59. fig. 3°. 
Compressed, with a low wing and a distinct angular line behind the summits. 
Hab. Yucatan: marshes near San Geronimo (Morelet 2*) ; Yucatan (Largilliert '). 
Var. planivalvis. 
Unio planivalvis, Morelet, Test. Noviss. ii. p. 24 (1851) *; Fisch. & Crosse, Miss. Scient. Mex., 
Moll. i. p. 560, t. 59. fig. 2°. 
Differs only in having the hinder part more prolonged. 
Hab. N. GuatEMALA: marshes near the Rio Usumacinta (Morelet + °). 
57. Unio scutulatus. 
Unio scutulatus, Morelet, Test. Noviss. i. p. 30 (1849) '; Fisch. & Crosse, Miss. Scient. Mex., 
Moll. ii. p. 561, t. 59. figg. 4-6, t. 67. fig. 6°. 
Compressed, with radiated sculpture and a very low wing-like prominence behind the summits. 
Hab. Yucatan : marshes near San Gerdnimo, with the preceding (Morelet | 2). 
Fischer and Crosse? distinguish two varieties—obliteratus, fig. 5, with very feeble, 
almost evanescent sculpture, and secadilis, fig. 6, with the sculpture of the hinder part 
considerably stronger, both from the same locality. 
58. Unio delphinulus. 
Unio delphinulus, Morelet, Test. Noviss. 1. p. 31 (1849)*; Hanley, Cat. Rec. Bivalv. p. 381, t. 23. 
fig. 60°; Sowerby, in Reeve’s Conch. Icon. xvi., Unio, t. 48. fig. 287°; Fisch. & Crosse, 
Miss. Scient. Mex., Moll. ii. p. 557, t. 68. figg. 2, 2a, 6*. 
Hab. N. Guatemata: marshes of the Rio Usumacinta (Morelet! +); Lake of Peten 
(Morelet' +). 
The localities “Tabasco,” in Paetel’s Catalogue, and “ Yucatan,” given by Hanley 2, 
