22 PALEONTOLOGY OF KENTUCKY. 



motion is produced either by the muscular feet surrounding their mouths, or 

 by two fins attached to the sides of their body, or, again, by the forcible ex- 

 pulsion of water through a tube, called the siphon. There are two orders of 

 Cephalopods : Dibranchiata and Tetrabranchiata, the first possessing two and 

 the second four branchiae. 



The first order includes the cuttle-fishes ; they are all naked, with the ex- 

 ception of the Argonaut. Their soft body is supported by an internal shell. 

 Plate I, figure 12, shows the complete animal of the Sepia officinalis ; and Plate 

 I, figure 16, is a Belemnite, or the internal shell of one of the dibranchiate 

 Cephalopods. Most all the species of this order are provided with an ink-bag, 

 the contents of which they discharge when pursued by their enemies. Of the 

 second order, the Tetrabranchiata, only very few species are living at the pres- 

 ent time. Plate I, figure 24, shows the Nautilus pompilius, the main repre- 

 sentative of the tetrabranchiate Cephalopods of the present seas. The oceans 

 of former ages were swarming with species of this order, fossil forms of which 

 are figured on Plate I, figures 1, 2, 18 and 19. Their shells are internally 

 divided into cells or chambers (see Plate I, figures 1 and 24), by a series of 

 partitions called the septa (see Plate I, figure 245), connected by a tube called 

 the siphon or siphuncle, shown by c, Plate I, figure 24. Only the last chamber, 

 called the body-chamber, is occupied by the animal, while the others are empty 

 and serve as air-chambers. In fossil specimens very often the outer shell is 

 removed, and the edges of the septa are seen (Plate I, figures 18 and 19), 

 which are called sutures. Their form serves to distinguish different genera ; 

 they are curved in Nautilus and Orthoceras, zigzag in Goniotites, or foliaceous 

 in Ammonite. 



The siphuncle is also of great importance in determining genera ; its shape 

 and location have to be noticed by the student of Palaeontology. Its shape is 

 so variable that it is impossible to give here a sufficient explanation of the 

 same, whilst its siphuncle may be located either in the center of the shell or 

 in its dorsal or ventral lines, or even on either of its sides. The opening of 

 the body-chamber is called its aperture, which, in different species and genera, 

 assume different shapes, and is generally closed by a calcareous plate, called 

 the operculum. 



GASTEROPODA. This class includes land and water-snails, which are either 

 naked or provided with a shell. They received their name because their loco- 

 motion is accomplished by the lower part of their body, which is provided 

 with strong muscles, and, by its contractions and expansions, serves them as a 

 foot. (See Plate I, figure 13.) Their shells are generally univalve, either 

 spiral or tubular, but a few are multivalve, the Chitons (see Plate I, figure 11). 

 In the spiral shells we have to notice the following parts : see Plate I, figure 



