Comprehensive Description

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Description

 

"Macrodon asperula n. s.

 

 

Shell small, white, slightly inequivalve, excessively inequilateral, byssiferous; with a thin brownish epidermis radiatingly streaked with thicker lines which fray out into hairs, the whole being very fugacious; hinge-line straight, the area long and narrow, the beaks situated just within the anterior third of the line; ligamentary area extremely narrow, beginning under the beak, extending close under the posterior areal carina obliquely to the posterior third of the hinge-line, where it first reaches the margin, flat, black, with several white longitudinal striæ; beaks well marked, not very prominent, with a faint depression extending from them to the basal margin, marking the position of the byssal gape; outline like that of Philippi's Arca aspera (Philippi, op. cit., II. Tab. XV. fig. 1 c), but much more inequilateral, the anterior end being smaller, the posterior larger and more extended, and the line of the base much more oblique, as would naturally follow from the above circumstances; sculpture of about a dozen (excluding those on the beaks) concentric sharp-edged subimbri­cate ridges, being old margins of the shell as it were forming varices, fainter and more close set on the beaks; with strong rounded radiating riblets (near the base about four to a millimeter) continuous only from one varix to another; posterior part of the cardinal border somewhat auriculated or compressed, anterior extreme rounded or moderately inflated, the anterior margin forming a little less than a right angle with the hinge-line, the posterior a little more than a right angle, the former slightly rounded to the base, where it is a little concavely excavated for the byssal gape, the latter first a little concave from the auricula­tion, then very broadly rounded out and passing without any angle into the base; right valve a little the smaller; interior white, both muscular scars double (by the splitting of the ends of the adductors); margin smooth, sharp; byssus slen­der, tufted at the distal end, light brown; hinge with six vertical teeth extending from the position of the beaks anteriorly, the first four small, vertical, gradually enlarging forward, the fifth largest, and with the sixth somewhat oblique; hinge between the beak and the ligamentary furrow edentulous, beyond that there are three horizontal teeth, the middle one largest extending posteriorly nearly parallel to the hinge-line and to each other. Lon. 8.5; alt. 5.0; max. diam. 3.25; lon. of hinge-line, 6.25 mm.

 

Station 33, 1568 fms., living; Station 19, 310 fms; Cape San Antonio, 1002, fms.

 

This is one of the most interesting of the Arcidæ obtained by the expedition. The type of the genus (Lycett, 1845) is a fossil of the lower Oölite, but there seems no reason for separating the recent shell from it generically or sectionally."

 

 

 

(Dall, 1908: 120-121)

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Creative Commons Attribution Non Commercial 3.0 (CC BY-NC 3.0)

© National Museum of Natural History, Smithsonian Institution

Source: Antarctic Invertebrates Website (NMNH)

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