Orthoceras (Gr. straight, and, horn), a fossil tetrabranchiate cephalopod, confined to the palaeozoic and early mesozoic periods, in which it played the part now taken by the carnivorous cattle fish. Though the shell is chambered, with a perforating siphon, as in the living nautilus, it is generally straight; in some allied genera, as lituites and cyrtoccras, the shell is partially coiled, but never so completely as in the mesozoic ammonites. It is likely that the animal could not get entirely within the outer chamber, and that from the buoyancy of the shell it mast have remained head downward. It attained a very large size, some being more than 10 ft. long, and as large round as a man's body. None have been found in strata later than the triassic age. They are allied to the nautilus on the one hand and to ammonites on the other. It is interesting to observe that these Silurian straight tetrabranchiate cephalopods gradually gave place to forms more and more coiled, till the tightly coiled ammonites of the mesozoic age appeared; then, as the type retrograded from this culminating point, the whorls began to unroll again, and such forms as ancyloccras, toxoceras, sca-phites, hamites, and haculites marked the extinction of the many-chambered cephalopods, whose principal present form is the nautilus.
1. Side view of fragment, showing septa. 2. Transverse section of same, showing the siphuncle, 3.