Entolium utukokense Imlay, 1961
IMLAY, R. W. 1961. Characteristic Lower Cretaceous megafossils from northern Alaska. United States Geological Survey Professional Paper, 335: 1-74, pls. 1-20. [p. 54, pl. 11, figs. 1-3, 5, 6, 8, 9]
1961 Entolium utukokense Imlay, 1961
R. W. Imlay, 1961, plate 11.
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«More than 100 specimens from northern Alaska are considered to belong to a single variable species. The valves are broadly ovate to nearly circular in outline. Some valves are slightly elongated posteroventrally. The left valves are moderately to strongly convex, have prominent umbones, and their surfaces bear irregular, broad concentric undulations. The right valves are slightly convex. Their surfaces are marked by closely spaced, regular concentric striae and by very widely spaced, narrow concentric furrows. The areas between the furrows are flat, or nearly flat. Some of the internal molds of the right valves bear radial striae, but such are not visible on the few specimens that retain the shell. The ears are nearly equal in size, rise slightly above the beak on the right valves and are slightly lower than the beak on the left valves. The surfaces of the ears bear fine growth lines and widely spaced furrows.
The interior of the right valve shows a resilium, a ligamental groove bounded by two ridges, and prominent crural ridges that are elevated at their ventral ends. On one specimen (pl. 11, fig. 3) the ligamental groove bears curved, transverse ridges. The dimensions of some left valves are as follows:
The left valve shown on plate 11, figure 8 is the most convex in the collections, but some left valves from northern Alaska are a little larger.
Entolium utukokense is distinguished from other described species of Entolium by its greater size and convexity. It appears to be most similar to E. alcesianum (McLearn) (1933a, p. 145, pl. 3, fig. 3), from the Clearwater formation of Alberta, but its left valve is much more convex and its left umbo is narrower and more prominent. The right valve of E. alcesianum (McLearn) is unknown and cannot be compared, therefore, unless it is represented by the holotype of E. irenense McLearn (1933a, p. 144, pl. 1, figs. 9, 10) of which species the left valve is unknown. The right valve of E. irenense appears to be less circular and less distinctly ribbed than any of the right valves of E. utukokense. All these species are assigned to Entolium rather than Syncyclonema because of their much larger size and because the ears on the right valve project dorsally above the hinge margin. Types.— Holotype, USNM 128706; paratypes, USNM 128707-128712. Occurrences.— Kukpowruk formation at USGS Mes. Iocs. 12480, 13721, 24454, 24458, 24460, 24466, 24469, 24482, 24483, 24487, 25786, 25790, 25791, 25801, 25806, 25810; Grandstand formation at Mes. Iocs. 20435, 20478, 24298, 24638, 25137; Tuktu formation at Mes. Iocs. 20403, 20405, 20407, 20432, 20484, 24621, 24622, 24623, 24626, 24637, 25121, 25133; Chandler formation at Mes. loc. 24432.» RALPH WILLARD IMLAY, 1961
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