Blakeoceras is a nautiloid  cephalopod from the Oncocerida

Oncocerida

The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian ,in which the connecting rings are thin and siphuncle segments are variably expanded...

 family Nothoceratidae with a curved shell that lived in shallow seas from the Silurian to the middle Devonian in what has become Europe.

The shell of Blakeoceras is a moderately expanded exogastric cyrtocone, the ventral side the outer curvature. Chambers in the phragmocone

Phragmocone

The phragmocone is the chambered portion of the shell of a cephalopod. It is divided by septa into camerae.In most nautiloids and ammonoids, the phragmocone is a long, straight, curved, or coiled structure, in which the camarae are linked by a siphuncle which determines buoyancy by means of gas...

 are short and wide, separated by straight, close spaced septa. Body chamber short. 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...

 ventro-marginal, with actinosiphonate fillings and concave segments.

Blakeocers is similar to Perimecoceras

Perimecoceras

Perimecoceras is a nothoceratid, nautilitoid order oncocerida, with a slowly expanding, compressed, cyrtoconic shell, known from the Upper Silurian of central Europe....

in general form but is more broadly expanded and has a proportionally shorter body chamber. Conostichoceras

Conostichoceras

Conostichoceras is an exogastric, breviconic oncocerid included in the family Nothoceratidae, known from the Middle Devonian of central Europe and Upper Devonian of Australia...

is less strongly curved and widens more laterally. Turnoceras

Turnoceras

Turnoceras is Devonian cephalopod belonging to the oncocerid family Nothoceratidae. Its shell is a broadly expanding exogastric cyrtocone with a flattened dorsum -on the inner concave side, unconstricted aperture, no hyponimic sinus, and ventral siphuncle -on the outer, convex side- composed of...

is more strongly curved and has a much greater expansion. Conostichoceras and Turnoceras are also more breviconic.

Blakeoceras is an early nothoceratid and may have been derived from Perimecoceras which has its origin in the Oncoceratidae

Oncoceratidae

Oncoceratidae, a family in the Oncocerida established by Hyatt, 1884, which is characterized by generally compressed, cyrtoconic and breviconic shells with an exogastric curvature such that the ventral profile is convex or more so than dorsal, and in which the siphucle is generally empty and...

, and is the likely source for the other exogastric nothoceratids with more greatly expanded shells.

Blakeoceras was probably a bottom feeder, based on its shell, that spent time, prowling, head down, over the sea floor.

-References-

• Sweet, W.C. 1964; Nautiloidea -Oncocerida; 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; Geological Society of America and University of Kansas press; Teichert and Moore (eds)
• Blakeoceras-Paleodb