FIGURE 3. A in Lower Jaw of Spathites (Ammonoidea: Acanthoceratoidea) from the Upper Cretaceous (Turonian) of New Mexico

FIGURE 3. A. Ventral view of the lower jaw (arrows) comprising two calcareous plates (= the aptychus). B. Close-up of the median hinge (arrow) between the two calcareous plates. C. Close-up of the broadly concave anterior margin (arrow) of the left plate. D. Close-up of the posterior margin (arrow) of the left plate near the median hinge showing the comarginal ribs. Scale bar = 1 cm.