Neithea youngi Myers, 1968
MYERS, R. L. 1968. Biostratigraphy of the Cardenas Formation (Upper Cretaceous), San Luis Potosi. Paleontología Mexicana, 24: 9-39, pls. 1-16 [p. 59, pl. 9, figs. 6-8]
1968 Neithea youngi Myers, 1968
R. L. Myers, 1968, plate 9.
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«Subtriangular outline, slightly inequilateral, very inequivalve. Height approximately equal to width. Right valve strongly and evenly convex, semicircular in profile. Hinge line short, less than half the width, almost straight. Beack is strongly incurved and passes beyond hinge line; angular distance from beak to ventral margin is almost 360°. Dorsal slopes are steep, partially overhang auricles. Auricles small, curved; each is separated from the main shell by o deep narrow sulcus.
Right valve ornamented with 16 principal ribs. Counting from either side, ribs 1-4-7-10-13-16 are higher and wider than the two ribs between each pair. Grooves between each rib are almost equaI. On a large shell, 38 mm high and 38 mm wide, the meanwidth of the six larger ribs is 2.2 mm, of the ten smaller ribs 1.7 mm, of the 15 grooves 1.5 mm. The ribs are about half a millimeter high. Ribs and furrows are generally round, only the smallest ribs are very slightly flattened. They are smooth, but may have 1 to 3 faint radial striae. Only in shells greater than 30 mm wide do these striae widen into very shallow grooves thereby dividing sorne of the principal ribs. There are 3 Iow thin secondary ribs between the first major rib and the anterior auricle, 4 Iow thin secondary ribs between last major rib and the posterior auricle. Remarks.— Neithea youngi belongs to the group of Neithea alpinus (d'Orbigny, 1847, which includes Pecten texanus Roemer (1852), Vola subalpina Böse (1910a), Neithea goorgetownensis Knicker (1918), Neithea budensis Knicker (1918), Neithea austinensis Knicker (1918), Neithea theodori Knicker (1918), and Pecten (Neithea) bexarensis Stephenson (1941). Neithea youngi is much more convex than most of these species and may be distinguished from all by its subequaI grooves separating simply rounded ribs of two distinct sizes, distributed as described above. Neithea youngi most closely resembles Pecten (Neithea) bexarensis in shape and ornamentation. However, N. youngi does not have the 2 to 5 developed subribs on each principal rib, and its grooves are relatively much wider. Neithea youngi has been collected from the Durania ojanchalensis Zone in railroad cuts between Km 419.5 and Km 419.7; and from the Arctostrea aguilerae Zone at Km· 414.5. RALPH LAWRENCE MYERS, 1968
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