Isognomon, SP., LIGHTFOOT, 1786

Hickman, Carole S., 2023, Paleogene marine bivalves of the deep-water Keasey Formation in Oregon, Part II: The pteriomorphs, PaleoBios 40 (5), pp. 1-51 : 21

ISOGNOMON ? SP.

FIG. 11

Discussion —A nacreous shell fragment from the upper member of the Keasey Formation preserves a portion of the ligament area of an isognomonid that clearly shows four narrow ligament pits and five broad, shallowly concave interspaces. The specimen preserves features that cannot be compared with Isognomon clarki , originally described as Pedalion clarki by Effinger (1938) from the coeval Gries Ranch Formation, or with the slightly older new genus and species described below from the middle member of the Keasey Formation. However, it is noteworthy in showing details of the ligament area that are not clearly preserved on any Paleogene isognomonid specimens from the Eastern Pacific margin of North America. The ligament area is clearly attached to nacreous aragonitic shell layers, and ontogenetically the width of the pits expands while the broad, concave interspaces contract in width.

Figured specimen — USNM 561801, length of fragment is 32 mm.

Keasey Formation Locality — USGS 15581. Upper member, Keasey Formation.

Figures

Figure 11. Isognomon? sp. Fragment of nacreous isognomonid ligament area with narrow ligament pits and five shallowly concave interspaces. USNM 561801. Scale bar=1 cm.