Mesoceras is a mid Silurian

Silurian

The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 discosorid

Discosorida

Discosorida is a unique order of cephalopods that lived from the beginning of the Middle Ordovician, through the Silurian, and into the Devonian. Discosorids are unique in the structure and formation of the siphuncle, the tube that runs through and connects the chambers in cephalopods, which unlike...

 from central Europe named by Barrande in 1877, with a short, anteriorly contracted body chamber and an aperture

Aperture (mollusc)

The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where part of the body of the animal emerges for locomotion, feeding, etc....

 that is a long transverse slit. (Teichert 1964). The cross section is slightly oval with the dorso-ventral dimension slightly less than the lateral. Sutures are straight and transverse, the siphuncle

Siphuncle

The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...

 is subcentral.

The relationship of Mesoceras to the other discosorids is unclear (Teichert 1964) but it is most likely derived from the Mandeloceratidae (Flower and Teichert 1957, Teichert 1964), perhaps from Ovacerina

References

• Flower and Teichert,1957; The Cephalopod Order Discosorida. Mollusca Article 6, University of Kansas Paleontological Contributions, July 1, 1957
• Teichert,C. 1964; Nautiloidea - Discosorida, in the 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; Geologocial Society of America and University of Kansas press, 1964.