together under the name of the “Tetrabranchiate” Cephalopods (Gr. tetra, four; bragchia, gill).  On the other hand, the ordinary Cuttle-fishes and Calamaries either possess an internal skeleton, or if they have an external shell, it is not chambered; their “arms” are furnished with powerful organs of adhesion in the form of suckers; and they possess only a single pair of gills.  For this last reason they are termed the “Dibranchiate” Cephalopods (Gr. dis, twice; bragchia, gill).  No trace of the true Cuttle-fishes has yet been found in Lower Silurian deposits; but the Tetrabranchiate group is represented by a great number of forms, sometimes of great size.  The principal Lower Silurian genus is the well-known and widely-distributed Orthoceras (fig. 55).  The shell in this genus agrees with that of the existing Pearly Nautilus, in consisting of numerous chambers separated by shelly partitions (or septa), the latter being perforated by a tube which runs the whole length of the shell after the last chamber, and is known as the “siphuncle” (fig. 56, s).  The last chamber formed is the largest, and in it the animal lives.  The chambers behind this are apparently filled with some gas secreted by the animal itself; and these are supposed to act as a kind of float, enabling the creature to move with ease under the weight of its shell.  The various air-chambers, though the siphuncle passes through them, have no direct connection with one another; and it is believed that the animal has the power of slightly altering its specific gravity, and thus of rising or sinking in the water by driving additional fluid into the siphuncle or partially emptying it.  The Orthoceras further agrees with the Pearly Nautilus in the fact that the partitions or septa separating the different air-chambers are simple and smooth, concave in front and convex behind, and devoid of the elaborate lobation which they exhibit in the Ammonites; whilst the siphuncle pierces the septa either in the centre or near it.  In the Nautilus, however, the shell is coiled into a flat spiral; whereas in Orthoceras the shell is a straight, longer or shorter cone, tapering behind, and gradually expanding towards its mouth in front.  The chief objections to the belief that the animal of the Orthoceras was essentially like that of the Pearly Nautilus are—­the comparatively small size of the body-chamber, the often contracted aperture of the mouth, and the enormous size of some specimens of the shell.  Thus, some Orthocerata have been discovered measuring ten or twelve feet in length, with a diameter of a foot at the larger extremity.  These colossal dimensions certainly make it difficult to imagine that the comparatively small body-chamber could have held an animal large enough to move a load so ponderous as its own shell.  To some, this difficulty has appeared so great that they prefer to believe that the Orthoceras did not live in its shell at all, but that its shell was