. Bulletins of American paleontology. Bulletin 332 (see Saunders. Jung, and Biju-Duval. 1986, text-fig. 23). which is cut in a thick coral reef approximately 2 km across (along the river) and at least 140 m in thick- ness. This canyon has until recently been more or less inacessible except by boat, but work on the Lopez- Angostura hydroelectric project in 1982-1983 opened up three access roads into this canyon from the east side. and. at the same time, created marvelous expo- sures for the entire section. That road upstream from the .Angostura canyon leading to "La Ventana" tunnel ex
. Bulletins of American paleontology. Bulletin 332 (see Saunders. Jung, and Biju-Duval. 1986, text-fig. 23). which is cut in a thick coral reef approximately 2 km across (along the river) and at least 140 m in thick- ness. This canyon has until recently been more or less inacessible except by boat, but work on the Lopez- Angostura hydroelectric project in 1982-1983 opened up three access roads into this canyon from the east side. and. at the same time, created marvelous expo- sures for the entire section. That road upstream from the .Angostura canyon leading to "La Ventana" tunnel exposed a most interesting section. Here, on the west bank of the river, there is a series of alternating cor- alliferous algal limestones and gray silty beds that bear a molluscan fauna most like that at Lopez. This area (Iocs. TU 1444-1447) was not investigated by the NMB team: therefore, a few words about the fauna are in order. The lowest locality in the section (loc. TU 1445) has elements that resemble both the Gurabo facies fauna [ Sconsia laevigata (Sowerby, 1850), Distor- sio sinullinnis (Sowerby. 1850)] and the Cercado facies fauna [ Meloiigena consors (Sowerby, 1850)], but there are also species that we have found nowhere else except at the Arroyo Lopez outcrop farther upstream ( Cerithiitm twriculum Gabb. 1873). As one goes up in the section the facies becomes increasingly shal- low, so that locality TU 1447, which is about 20 m abo\e river level on the access road and stratigraph- ically as much as 50 m above locality TU 1445. is the site of a number of unique species, all of which indicate very shallow or even brackish water. These include numerous specimens (13 in all) of .\felongena ortha- cautha Pilsbry and Johnson. 1917 (see Pilsbry, 1922, pi. 28. figs. 13-16); and rare examples of Potamides pnsinaticiis (Gahh. 1873) (see Pilsbry, 1922, pi. 29, fig. 12) and Terebmlia dentilabhs (Gabb. 1873) (see Pils- bry. 1922, pi. 29. figs. 6, 7). In addition, th
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