Euvola mimyum (Woodring, 1982)
WOODRING, W. P. 1982. Geology and paleontology of Canal Zone and adjoining parts of Panama. United States Geological Survey Professional Paper, 306-F: 1-759, pls. 83-124. [p. 587, pl. 122, figs. 2-3; pl. 123, fig. 8; pl. 124, fig. 4]
1982 Amusium mimyum Woodring, 1982
W. P. Woodring, 1982,
plates 122, 123. |
«Of small to medium size, thin-shelled, height generally little less than length, exceptionally little more. Both valves slightly convex. Umbonal area of both valves sculptured with riblets, extending 5 to 16 mm (generally not more than 12 mm) from tip of umbo. Auricles not ribbed; right auricles generally very slightly more depressed than left; byssal notch very shallow, imperceptible on many small valves. Interior lirae generally extending far upward from ventral margin; those of right valve generally weakly grouped into 15 to 17 pairs, or equidistant; those of left valve generally equidistant, or weakly grouped into pairs. A pair of cardinal crura and a pair of elongate auricular crura, ending in a tubercle.
Length 69.9 mm, height 66.5 mm, convexity 6 mm (type, largest specimen). Length 47.5 mm, height 48.6 mm, convexity 6 mm (figured small left valve). Type (right valve) : USNM 647134. Type locality: 206 (USGS 16937, Caribbean coast near mouth of Rio Pifia; roadcut on west side of river about 90 m west of road fork, Panama), Chagres sandstone. Amusium mimyum, which occurs in the Chagres sandstone, is the youngest of the umbonally ribbed species and, with the exception noted in a following paragraph, is also the smallest. The distribution of the 65 valves, and the number and maximum length at each locality are shown in the following table: Distribution of Amusium mimyum, number of valves,
and maximum length
It is apparent that the only relatively large specimens are from locality 206, the type being the largest of all. The type is a right valve, but the ventral part of the anterior auricle is defective. The very shallow byssal notch is shown on a smaller right valve from the type locality (pI. 124, fig. 4), a valve that is comparable in size to the largest from other localities. Owing to the equal convexity of the valves, the shallowness of the byssal notch, and the slight difference in depression of the auricles, many small valves are indistinguishable as right or left.
A small right valve (length 42 mm) from the early Miocene Anguilla formation of the island of Anguilla, originally in the Guppy collection, was identified by Dall as A. lyonii, cited under the preceding species. Cooke (1919, p. 142, pI. 13, figs. 1a, 1b, 2) also identified as A. lyonii two additional Anguillan specimens. The shell of A. mimyum is thinner than that of the Anguillan fossils, doubtless correlated with its deeper water habitat, and its auricles are shorter and narrower than those of Cooke's larger specimen (his figs. 1a 1b), which has an estimated restored length of 57 mm. Whether the Anguillan fossils represent a small form of A. lyonii is indeterminable at present. Toula's thin-shelled Amusium from Tehuantepec also was identified by Dall as A. lyonii, but Cooke thought it was not the same as the Anguillan species. It is represented in USNM collections by an incomplete left valve (135059, estimated restored length 50 mm), which is assigned to A. mimyum. Occurrence: Chagres sandstone (late Miocene or Pliocene), localities 199, 201, 202, 206, 208. Deposits of late Miocene age, Tehuantepec area, Mexico.» WENDELL PHILLIPS WOODRING, 1982
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