''Benacoceras'' is a strongly ribbed, evolute
ammonite
Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...
from the Late Jurassic, coiled so that all whorls are exposed. Ribbing is biplicate, with very short secondaries. The venter, the outer rim, is smooth, at least on the outer whorl.
References
;Notes
;Bibliography
*Treatise on Invertebrate Paleontology, Part L; Ch. Mesozoic Ammonoidea. Geological Soc of America and U Kansas Press, R.C Moore (ed)
Jurassic ammonites
Kimmeridgian life
{{Ammonite-stub