Amussium [sic] sewelli Knudsen, 1967
KNUDSEN, J. 1967. The deep-sea Bivalvia. The John Murray Expedition 1933-34, Scientific Reports, 11 (3). Publications of the British Museum (Natural History), 657: 237-343, pls. 1-3, text-figs. 1-38 [p. 279, pl. 2, figs. 1, 2; text-figs. 18A, 18B]
«Amussium sewelli n. sp.
Text fig. 18A, B; pl. 2, figs, 1, 2. MATERIAL: ''John Murray" Expedition: St. 118, Zanzibar area, 4°05'54"S., 41°10'12"E.; depth: 1789 m; bottom: globigerina ooze; bottom temperature: 3.0°C; gear: Agassiz trawl. – 2 specimens.
DIAGNOSIS: An Amussium having 11 costae and a mantle edge provided with an external row of small bulbous tentacles and two internal rows of larger pointed tentacles of varying sizes. DESCRIPTION: The shell is transparent and somewhat higher than long, the left valve being somewhat larger and more convex than the right. The left valve has a distinct and regular concentric sculpture; the central part of the valve has a somewhat less distinct radiating sculpture which tends to become obsolete towards the periphery. The right valve has only a regular concentric sculpture. In the type the right valve has a faint brown colouration, whilst the left is completely white (in the paratype both valves are white). The auricles are relatively small, nearly equal in size and provided with a transverse sculpture which is slightly more distinct on the anterior auricle than on the posterior. The prodissoconch is about 290 μ long. Eleven costae are present. They are distinctly marked off and gradually become broader towards the periphery. The central costae are devoid of terminal nodules, while indistinct nodules are found on the anterior and posterior costae. The soft parts: the middle lobe of the mantle has three rows of tentacles. The tentacles of the external row are very small and bulbous distally. The tentacles of the two inner rows are considerably larger, pointed and of varying sizes. The velum is very well developed and without tentacles. The mantle edge, as well as the remaining part of the body is devoid of pigment. The foot is very small and bulbous distally. DIMENSIONS IN MM: Type: right valve: 27 X 32; left valve (approximately): 36 X 34.
RELATIONSHIP: Smith (1906; p. 255, 256) described two species of Amussium, viz., A. investigatoris and A. manaricum. The former from "Investigator" St. 248, 410-519 m depth; the latter from "Investigator" St. 322 and 333, 691 and 733 m depth. These species have never been figured. As far as can be judged from the descriptions, the present species seems to be different from these. A. investigatoris has 10 costae, while A. manaricum has 15. A. sewelli, with 11 costae, seems to be rather close to A. watsoni (below), which also has 11 costae. These two species differ in the shape of the auricles, and A. sewelli does not have the reflection of the posterior edge of the posterior auricle found in A. watsoni. In addition, these two species differ in the arrangement of the tentacles of the mantle edge, A. watsoni lacking the small bulbous tentacles. The species is named for Dr. R. Seymour Sewell, the leader of the "John Murray" Expedition. HOLOTYPE: B.M.(N.H.) Reg. No. 196641 W. PARATYPE: B.M.(N.H.) Reg. No. 196642 W.» JØRGEN KNUDSEN, 1967
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J. Knudsen, 1967, plate 2.
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