112 HISTORICAL PALEONTOLOGY. 



which it is organically connected by muscular attachments. 

 The head is furnished with long muscular processes or "arms/' 



Fig. 54. Different views of Maclnrca crenulata, Quebec Group, Newfoundland. 

 (After Billings.) 



and can be protruded from the mouth of the shell at will, or 

 again withdrawn within it. We learn, also, from the Pearly 

 Nautilus, that these animals must have possessed two pairs of 

 breathing organs or " gills ; " hence all these forms are grouped 

 together under the name of the " Tetrabranchiate " Cephalo- 

 pods (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 num- 

 ber of forms, sometimes of great size. The principal Lower 

 Silurian genus is the well-known and widely-distributed Ortho- 

 ceras (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 per- 

 forated 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 cf rising or sinking in the water by driving additional 

 fluid into the siphuncle or partially emptying it. The Ortho- 



