Spondylus munitus Stephenson, 1941
STEPHENSON, L. W. 1941. The larger invertebrate fossils of the Navarro Group of Texas. University of Texas Publication, 4101: 1-641, pls. 1-95. [p. 139, pl. 24, figs. 8, 9]
1941 Spondylus munitus Stephenson, 1941
L. W. Stephenson, 1941, plate 24.
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«Shell of moderate size, subovate in outline, strongly convex, subequilateral, strongly inequivalve. The two valves differ markedly in form and sculpture. The left valve is depressed convex; shell destroyed except the ears, and parts of the dorsal slopes; it is thin about the beak, and thickens toward the margins; the portion of shell remaining on the posterodorsal slope exhibits 7 low ribs of somewhat irregular trend, narrower than the interspaces and supporting a few scattered low, blunt spines; the posterior ear is of moderate size and is marked by 3 or 4 very obscure ribs, and along its lower margin is a row of 6 very unequal closely spaced spines; the 3 outer spines are partly broken away, but are strong and prominent, while the 3 inner ones are smaller and, though broken, are probably shorter; the anterior ear is slightly smaller and is wanting in radial ribs; one low strong rib on the upper border of the main shell surface, borders the lower margin of this ear; impressions on the internal mold indicate that the left valve is completely ornamented with numerous, radiating ribs of unequal size and spacing, the ribs in general being narrower than the interspaces; the mold also exhibits numerous, fine radiating lines between and on the rib impressions; the beak is small, incurved, direct, and projects only slightly above the hinge line. About the posterior half of the right valve is preserved and exhibits a series of prominent, wall-like, concentric ridges rising nearly at right angles to the surface, or tilted slightly upward, averaging about 1 millimeter in thickness and spaced somewhat irregularly but reaching a maximum of 8 mm. apart toward the outer margin; all these ridges are more or less broken away, but one can be seen to rise fully 5 mm. above the surface; the surface of the shell between the concentric ridges is marked by low, obscure radiating ribs, 11 or 12 to the centimeter, wider than the shallow interspaces, and the whole surface exhibits fine radiating lines; both ribs and lines are reflected on the internal mold; the beak is strongly incurved and strongly prominent; its tip is obscured by the attachment of the shell over a large area to one or more irregular shell fragments; the ears are small and are ornamented only with growth lines. The hinge is straight and about 14 mm.long. With the exception of the rib markings on the internal mold, the internal features are not observable.
Dimensions of the holotype: Right valve, length 23 mm., height 32 mm., thickness about 22.5 mm.; left valve, length 22 mm., height 26.7 mm., convexity about 4.5 mm. The only American Upper Cretaceous species at all like this one is Spondylus gregalis (Morton), from the Navesink marl, New Jersey. It is a larger, more elongated and less ventricose species, and has flatter, more irregular, concentric laminae. Holotype.— U.S.N.M. no. 76452. Distribution in Texas.— Navarro group, Corsicana marl: 1 3/10 miles north of McQueeney (15524).» LLOYD WILLIAM STEPHENSON, 1941
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