. Bulletin of the British Museum (Natural History). . 449 Figs 448-451 Miocene fossils from the Cuenca Basin, Ecuador; Bristow Colin. 448-450, from well-bedded mudstone at Loc. CRB 11, Loyola Formation; all x 10. 448, 449, GG19814/1, 2; ?Dyris sp., probably the species listed as Lyrodes sp. in Bristow & Parodiz (1982: 7). 450, GG19814/3; indeterminate Hydrobiidae, ?Dyris sp., probably the species identified as Hydrobia ortoni in Bristow & Parodiz (1982: 7). 451, LL27817; Corbicula (Cyanocyclas) cojitamboensis Palmer. The specimen identified as Erodona iquitensis in Bristow &a
. Bulletin of the British Museum (Natural History). . 449 Figs 448-451 Miocene fossils from the Cuenca Basin, Ecuador; Bristow Colin. 448-450, from well-bedded mudstone at Loc. CRB 11, Loyola Formation; all x 10. 448, 449, GG19814/1, 2; ?Dyris sp., probably the species listed as Lyrodes sp. in Bristow & Parodiz (1982: 7). 450, GG19814/3; indeterminate Hydrobiidae, ?Dyris sp., probably the species identified as Hydrobia ortoni in Bristow & Parodiz (1982: 7). 451, LL27817; Corbicula (Cyanocyclas) cojitamboensis Palmer. The specimen identified as Erodona iquitensis in Bristow & Parodiz (1982: 10, 31, but not that illustrated in their fig. 8); Loc. CRB 7, Basal Loyola Formation. Latex casts; a, external view of left valve, x 4 (interpreted as right valve in Bristow & Parodiz); b, internal view of left valve, x 2. See p. 315. common C. cojitamboensis and S. bibliana, is likely to be fairly similar in age to the richer Loyola Formation. In an area of predominantly coarse sedimentation, the strata inter- vening between these two formations may well have been deposited fairly rapidly, so there is no real reason to postulate that the Mangan Formation is much younger than the Loyola. Neritina can occur in fresh water, but is the only genus in this fauna which also has brackish and marine species. The remaining taxa are of exclusively fresh-water families. Cuenca Basin, Azogues and Guapan Formations (dated as Middle Miocene by Bristow & Parodiz, 1982) They (1982: 7, 11-12) recorded only three molluscan species from these formations. Both Corbicula cojitamboensis and Aylacostoma cf. dickersoni ( = Longiverena cf. eucosmia herein) are known from other formations in the Cuenca Basin, whilst Diplodon sp. belongs to a genus common in the basin. All three indicate a fresh-water environment. Cuenca Basin, Loyola Formation (dated as upper part of Lower Miocene by Bristow & Parodiz, 1982) Of particular importance is the radiometric dating by Snelling (
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