Ephippioceratidae is a family of clydonatilacean

Clydonautilaceae

The Clydonautilaceae is a superfamily within the nautiloid order Nautilida characterized by smooth, generally globular, shells with nearly straight sutures, in early forms but developing highly differentiated sutures in some later forms...

 nautilids

Nautilida

The Nautilida constitute a large and diverse order of generally coiled nautiloid cephalopods that began in the mid Paleozoic and continues to the present with a single family, the Nautilidae which includes two genera, Nautilus and Allonautilus, with six species...

 with shells as in the Liroceratidae

Liroceratidae

Liroceratidae is an extinct family of nautilids, shelled marine molluscs, belonging to the Clydonautilaceae, consisting of generally smooth, involute, nautiliconic forms with a small umbilicus...

 but with sutures that have deep ventral and dorsal saddles. This group, which contains two genera, Ephippioceras and Megaglossoceras, has a range from the Mississippian to the Lower Permian.

Ephippioceras, which has the full range of the family, has a broad, narrowly peaked ( V-shaped) ventral saddle and may have been derived from Liroceras

Liroceras

Liroceras is the type genus of the clydonautiliacean family, Liroceratidae, and is characterized by a rapidly expanding, subglobular, nautiliconic shell with a reniform whorl section, small umbilicus, essentially straight sutures, and a siphuncle with a variable but not marginal position.Liroceras...

early in the Mississippian. Megaglossoceras from the Pennsylvanian

Pennsylvanian

The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...

 of North America, with its large, broadly arched, tongue-like ventral saddle is an obvious offshoot of Ephippioceras. Both are subglobular and involute with a reniform whorl section. Ephippioceras has been found in North America, Europe, and China.

References

• Bernhard Kummel, 1964. Nautiloidea -Nautilida; Treatise on Invertebrate Paleontology
  Treatise on Invertebrate Paleontology
  The Treatise on Invertebrate Paleontology published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and covering every phylum, class, order, family, and genus of fossil and...
  
  , Part K. Geol Soc of America and Univ of Kansas press, R.C. Moore (ed)