THE LOWER SILURIAN PERIOD. 115 



erous 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 sup- 

 posed 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 Ortho- 

 ceras 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 center 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 gradu- 

 ally expanding towards its mouth in front. The chief objec- 

 tions to the belief that the animal of the Orthoceras was essen- 

 tially 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 cer- 

 tainly 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 

 an internal skeleton similar to what we shall find to exist in 

 many of the true Cuttle-fishes. There is something to be said 

 in favor of this view, but it would compel us to believe in the 

 existence in Lower Silurian times of Cuttle-fishes fully equal 

 in si/,e to the giant " Kraken " of fable. It need only be 

 added in this connection that the Lower Silurian rocks have 

 yielded the remains of many other Tetrabranchiate Cephalo- 

 pods besides Orthoceras. Some of these belong to Cyrtoceras, 



