Didymoceras Hyatt, 1894

KENNEDY, W. J., LANDMAN, N. H., COBBAN, W. A. & JOHNSON, R. O., 2000, Additions to the Ammonite Fauna of the Upper Cretaceous Navesink Formation of New Jersey, American Museum Novitates 3306, pp. 1-31 : 15

Genus Didymoceras Hyatt, 1894

TYPE SPECIES: Ancyloceras? Nebrascensis Meek and Hayden, 1856 (p. 71); by original designation by Hyatt (1894: 574) .

Didymoceras cf. D. draconis (Stephenson, 1941) Figure 5I, J

compare:

Nostoceras? draconis Stephenson, 1941: 413 , pl.

82, figs. 5–9.

DESCRIPTION: MAPS 2040b1 comprises just over one whorl of a helix with a maximum diameter of 57.4 mm; the maximum whorl height is 20.3 mm. The helix appears to have been low so that the whorls were in contact; the upper whorl surface is feebly concave to accommodate the base of the preceding whorl. The adapical half of the fragment is phragmocone, the remainder, body chamber. Narrow delicate ribs are markedly rursiradiate on the upper whorl face, but strengthen and cross the juncture of the upper and outer whorl faces in a feeble concavity, where they may bifurcate. The ribs are even, slightly prorsiradiate, and weakly concave on the outer whorl face; there are occasional intercalated ribs, yielding a total of approximately 85 ribs per whorl. The ribs join in pairs at relatively coarse, transversely elongated, flat­topped tubercles at the juncture of the outer and lower whorl faces, or intercalate between the tubercles. There are 27–30 tubercles per whorl. Single coarse ribs link to much smaller, sharper tubercles on the out­ er part of the lower whorl face, with single nontuberculate ribs between them. The ribs are strong and feebly convex across the remainder of the lower whorl face.

DISCUSSION: The position of the tubercle rows, one at the juncture of the outer and lower whorl faces and the other on the outer part of the lower whorl face, combined with fine, dense ribbing and large size, distinguish this specimen from all others in the Navesink fauna. This specimen most closely resembles Didymoceras draconis (Stephenson, 1941) (p. 413, pl. 82, figs. 5–9).

OCCURRENCE: Lower phosphatic layer, basal Navesink Formation, Atlantic Highlands, New Jersey.

Figures

Fig. 5. A–E, K–O. Nostoceras (Nostoceras) approximans (Conrad, 1855). A, B. MAPS A2027a2, macroconch; C–E. holotype, ANSP 12861, microconch, White River, Arkansas; K–M. MAPS 2027a1, microconch; N, O. USNM 445246, pathological microconch. F–H. Exiteloceras rude, n. sp., holotype, USNM 433778, Big Brook, near Marlboro, Monmouth County. I, J. Didymoceras cf. D. draconis (Stephenson, 1941), MAPS A2040b1. All specimens are from the lower phosphatic layer, Navesink Formation, Atlantic Highlands, New Jersey, unless otherwise stated. All figures are X1.