. Bulletin of the British Museum (Natural History). PEBASIAN MOLLUSCAN FAUNAS 321. 442a 442b Figs 441-442 Guianadesma sinuosum Morrison. Recent; Surinam. BMZD 1984228; same locality and collection as Fig. 440. 441, a-c, all left valve; a, external, x ; b, c, internal, x , x 5. d-f, all right valve; d, external, x ; e, f, internal, x , X 5. 442, a, b, left and right valves, internal views, x margin. The ventral commissure is flexed, more strongly in the right valve than in the left. In all the BMZD specimens the umbones are badly eroded. In some cases other areas have
. Bulletin of the British Museum (Natural History). PEBASIAN MOLLUSCAN FAUNAS 321. 442a 442b Figs 441-442 Guianadesma sinuosum Morrison. Recent; Surinam. BMZD 1984228; same locality and collection as Fig. 440. 441, a-c, all left valve; a, external, x ; b, c, internal, x , x 5. d-f, all right valve; d, external, x ; e, f, internal, x , X 5. 442, a, b, left and right valves, internal views, x margin. The ventral commissure is flexed, more strongly in the right valve than in the left. In all the BMZD specimens the umbones are badly eroded. In some cases other areas have been worn to such an extent that holes have appeared in the shell. The periostracum is concentrically wrinkled, par- ticularly cMteriorly, whilst posterior to the flexure it is formed into numerous radiating rib-like thickenings. Concentric and radiating elements form a net-like pattern over much of the shell surface. This is confined to the periostracum: there is no evidence that the patterning extends to the calcareous shell which is unsculptured except for slightly rugose growth lines. Remarks. Morrison live from rocks in mi Kartabo Point, near Essequibo District, C 50) that paratypes we Museum, Pittsburgh, byssally attached to current, but absent (1943: 51). collected the holotype, USNM 53691, dstrcam of the Cuyuni River, opposite the junction with the Mazaruni River, juyana in July, 1925. He stated (1943: re present both in USNM and Carnegie The species was moderately abundant, igneous rocks and gravel in the river from rocks surrounded by mud bars Altena (1969: 29; 1971: 83) reported finding the species sometimes byssally attached in old borings of tercdinids in the Surinam River. The strong flexure of the ventral commissure and the rather modioliform shell outline with a concavity in the ventral margin are entirely in keeping with a byssally attached mode of life, and it is possible that the more elongate, Lithophaga-Wke individuals are those which lived in tubes. The o
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