Armenoceras richmondensis

Phylum: Mollusca
Class: Cephalopoda
Order: Actinocerida
Family: Armenoceratidae
Genus: Armenoceras
Species: Armenoceras richmondensis (Foerste, 1917)

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Formerly Triptoceras (Lambeoceras) richmondensis
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Stratigraphic Occurrences


Geographic Occurrences

Map point data provided by iDigBio.
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Sequences (Formations)

  • C5 Sequence (Liberty, Waynesville)

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Identification in Hand Sample

  • Shells usually fragmentary and flattened
  • Siphuncle is always much wider than high, strongly depressed in section
  • Has a remarkably small septal foramen
  • Apparent expansion of the segments in the camerae is slight
  • Radial canals are nearly normal to the central canal

Armenoceras richmondensis from the Liberty Formation of Clifty Falls, Indiana. (CMC 24472).

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Flower (1946):

  • This form is represented by shells usually fragmentary and flattened but which show that this species had a phragmocone rather similar to that of A. madisonense but differing in the more central position of the siphuncle. The holotype, selected as the least distorted although not the most complete specimen, is a portion of a phragmocone 55 mm. in length, increasing in width from 40 mm. to 50 mm. in a length of 45 mm. The shell height, decreased partly by weathering and partly by slight pressure, is 38 mm. adorally, and was probably nearly as great basally. At the adoral end of the shell the siphuncle is 14 mm. high and 21 mm. wide at the septal foramen, is 7 mm. from the ventral wall and about 18 mm. from the dorsum. At a shell width of 50 mm. the septa are equal in curvature to one and one-half camerae. Seven and a half camerae occur in a length equal to the adoral shell width.
  • The siphuncle is always much wider than high, strongly depressed in section. The segments in section show a remarkably small septal foramen, actually 9 mm. in width where it is apparently 14 mm. in width, for the septal necks are recumbent and broadly adnate to both ends of the connecting ring, extending far within the apparent minimum diameter of the siphuncle, where they are obscured by the recrystallized annulosiphonate deposits. The apparent expansion of the segments in the camerae is slight, one increasing from 18 mm. to 23 mm., where the length of the segment is 7 mm.
  • In section, the siphuncle shows that the septa continue far within the apparent septal foramen, so that where the apparent minimum width of the segment is 18 mm., the septal foramen was actually only 9 mm. wide. The condition of the necks and brims is obscured in most cases by recrystallization of the calcite surrounding and composing them, but at several points the sections show that the brim is strongly recurved, but actually does not become recumbent, and fails to touch the free part of the septum. Also, the adoral portion of the connecting ring remains free from the septum, although adapically a broad area of adnation is developed. The radial canals are nearly normal to the central canal, but their arrangement cannot be made out clearly. The camerae bear only very thin hyposeptal and episeptal deposits which are unusually thin throughout, even where the segments of the siphuncle contain mature annulosiphonate deposits.

Foerste (1917)

  • Othroceracone greatly compressed, transverse section (figure 2, place III) lenticular, the lateral edges acute. Radius of curvature across the middle of the septa slightly more than half of the chord connecting the ends of the arc formed by the septa; for instance, the radius of curvature of the lowest septum preserved in the specimen figured is 28 millimeters. Along the ventral side of the shell the transverse curvature of the septa is somewhat greater than the dorsal side, causing the suture line on this side to rise more rapidly than on the dorsal side within a distance of 5 to 7 millimeters of the acute lateral angles of the shell, beyond which there is slight reversal of curvature. This causes the lateral parts of the septa to slope moderately upward from the dorsal toward the ventral side. The dorsal and ventral sides of the orthoceracone are about equally convex. The radius of curvature equals about seven-tenths of the chord connecting the ends of the arc formed by the curvature. The lateral sides diverge at an angle of about 7 degrees. The antero-posterior diameter of the shell is 20 millimeters where its width is 53 millimeters. From this point upward 10 chambers occur in a length of 48 millimeters. The siphuncle, or at least the cast of its interior, is nummuloidal, the lateral diameter being 7 millimeters where the shell is 53 millimeters wide. The more exact structure of the siphuncle has not been determined.

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