. Bulletin of the British Museum (Natural History). 286 C. P. NUTTALL. Figs 347-348 Mytilopsis scripta (Conrad). Pebasian; Canama, Peru; originally described as Dreissena acuta Etheridge; Barrington Brown Colin. 347, LL27913 {ex BMPD 97730); holotype of Dreissena acuta Etheridge, figured by Etheridge (1879: pi. 7, fig. 1); a, b, left valve, x 3. 348, LL27914; right valve found with LL27913, but not mentioned by Etheridge; a, side view, x 5; b, front view, x 8; c, interior, x 5; d, interior, detail showing septum and apophysis, x 20; e, umbonal region viewed from posterior end of shel
. Bulletin of the British Museum (Natural History). 286 C. P. NUTTALL. Figs 347-348 Mytilopsis scripta (Conrad). Pebasian; Canama, Peru; originally described as Dreissena acuta Etheridge; Barrington Brown Colin. 347, LL27913 {ex BMPD 97730); holotype of Dreissena acuta Etheridge, figured by Etheridge (1879: pi. 7, fig. 1); a, b, left valve, x 3. 348, LL27914; right valve found with LL27913, but not mentioned by Etheridge; a, side view, x 5; b, front view, x 8; c, interior, x 5; d, interior, detail showing septum and apophysis, x 20; e, umbonal region viewed from posterior end of shell showing apophysis hanging below dorsal margin of septum, x 20. Distribution. Late Oligocene, western Peru; ?Miocene, Magdalena Valley, Colombia; Late Caenozoic, La Tagua; Late Caenozoic, Pebasian (Pebas, Pichana, Iquitos, Canama, Tres Unidos). DIAGNOSIS. Mytilopsis with a straight dorsal margin and prominent umbonal ridge; shell outline often markedly tri- gonal, but sometimes less regular; apophysis similar to that of M. sallei, relatively lanceolate, adjacent and parallel to dorsal margin of shell. Colour patterning variable, but sometimes prominent. Description. The dorsal margin is almost straight and is usually about two-thirds of the total shell length, though sometimes, as in the case of Conrad's type illustration (1874a: pi. 1, fig. 16), it is much shorter. The outline is rather variable. At one extreme there arc specimens such as LL27956, rather similar to Conrad's type illustration; at the other, the shell is much more triangular as in the holotype of D. acuta Etheridge, the type illustration of M. cira Pilsbry & Olsson and the two La Tagua specimens. Intermediate stages occur. Examples from Iquitos covering the whole range of variation arc illustrated by de Greve (1938: pi. 9). The umbonal angle is also variable. In anterior view, the posterodorsal and anteroventral slopes meet at about right angles to form a prominent umbonal ridge. The ridge itself is sometim
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