Macrocephalites is a genus of the stephanoceratacean

Stephanocerataceae

Stephanocerataceae is a superfamily of middle Jurassic ammonoid cephalopods within the order Ammonitida containing diverse forms, generally with sharp ribbing and complex suture lines...

 ammonite

Ammonitida

The Ammonitida is an order of more highly evolved ammonoid cephalopods from the Jurassic and Cretaceous time periods, commonly with intricate ammonitic sutures....

 family Macrocephalitidae, diagnostic of the Callovian

Callovian

In the geologic timescale, the Callovian is an age or stage in the Middle Jurassic, lasting between 164.7 ± 4.0 Ma and 161.2 ± 4.0 Ma. It is the last stage of the Middle Jurassic, following the Bathonian and preceding the Oxfordian....

 stage of the Middle Jurassic

Jurassic

The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

. Three subgenera, Dolikephalites, Kamptokephalites, and Pleurocephalite are recognized in addition to Macrocephalites itself, with Indocephalites tentatively included as the fourth.

Diagnosis Macrocephalites s.s.

Macrocephalites itself has a strongly ribbed, involute, subglobose to discoidal shell with a somewhat small open umbilicus, compressed whorl section, higher than wide, and rounded venter. Early whorls are more strongly ribbed than later, which become smoother, ending with a smooth body chamber. Primary ribs, which emanate radially from the umbilicus, bifurcate and sometimes trifurcate mid flank and cross the venter uninterrupted. Shells grew to be large, even gigantic. Distribution is widespread.

Subgenera

Dolikephalites is similar in general form to Macrocephalites but has fine ribbing that continues to the end and is never large.

Kamptokephalites is more globose than Macrocephalites proper, with a depressed whorl section, wider than high, broadly arched venter, coarse plicate ribbing and wider umbilicus.

Pleurocephalites also has a broad shell with depressed whorl section and sharp ribbing that continues to the end. Primary ribs on the inner part of the whorls slope dorso-ventrally to the rear.

Indocephalites is broad and coarsely ribbed in early growth stages but becomes smoother and more compressed in the adult.