278 PALAEOZOIC TIME — DEVONIAN AGE. 



III. General Observations. 

 Geography. — In the first epoch of this period, that of the Cauda- 

 Galli grit, the beds were, as a body, more easterly in position over 



called shagreen, and may sometimes be seen to be composed of minute rhombic 

 or angular pieces, each rising into a point at centre. 



II. Ganoids. — Include the modern and ancient Gars (figs. 517-519), and the 

 Sturgeon, having (1) the skeleton cartilaginous or bony; (2) the gills as in 

 ordinary fishes ,• (3) the body covered with bony plates or scales (figs. 474, 

 475), which are usually shining or enamel-like in surface; (4) the optic nerves 

 not decussating. The name Ganoid is from yauog, shining, and alludes to the 

 scales. 



III. Common or Osseous Fishes, or Teliosts. — The Perch, Salmon, and all 

 common fishes are here included. (1) The skeleton is bony, as the name Teliost — 

 from nAEioj, complete, and oanov, bone — implies; (2) the gills have one margin 

 free; (3) the scales covering the body are membranous; (4) the optic nerves 

 decussate. The Cycloids of Agassiz have the scales unarmed with sharp points 

 (fig. 476) ; while the Ctenoids (from ictus, a comb) have them armed (fig. 477) : 

 but this subdivision is not a natural one. 



1. Selachians — The Selachians are divided into three groups, the Squa- 

 loids, or Sharks, the Bays, and the Chimseroids. The Squaloids have an elon- 

 gate body, with the gill-openings lateral ; the Rays, a "broad, flat body, with the 

 gill-openings in the ventral or under surface; the Chimgeroids, only one gill- 

 opening, besides other peculiarities. 



The Squaloids include, — 



(1.) The True Sharks, or Squalodonts, having sharp-edged teeth (figs. 465-467), 

 and the mouth on the under surface of the head (fig. 462). 



(2.) The Hybodonts, having teeth nearly like the preceding, but with the edge 

 less acute : they are intermediate between the Squalodonts and the Cestracionts 

 (figs. 468, 469). 



(3.) The Cestracionts, having a rough pavement of bony and usually enamelled 

 pieces in the mouth, and the mouth situated at the extremity of the head ; fig. 

 464, Cestracion Philippi or "Port Jackson Shark," Australia; 470, side-view of 

 mouth, showing the pavement or grinding surface of lower jaw, with the pointed 

 teeth at the opening; 471, 472, different views of the pavement-pieces in the 

 Cestraciont genus Acrodus. In the genus Cochliodus the number of pavement- 

 pieces is very small, and they are proportionally large (figs. 546, 547), besides 

 having a spiral twist. These Cestraciont mouths were well fitted for masti- 

 cating Ganoids and shell-fish. 



The Chimmroids, including the living Chimserse and several extinct species, 

 have two to four osseous plates to either jaw in place of teeth. 



Among the Rays, Myliobates and species of some related genera use their large 

 pectoral fins in swimming, instead of the tail, and the motion is much like that 

 of flying through the water, — so that they arc sometimes called Sea-Eagles. The 

 mouth in this group is paved with four- or six-sided plates, evenly and neatly 

 joined. 



