Devonian Age of Fishes. 81 



species between the Chenmng and the Hamilton. Land plants multi- 

 ply, and the marine shell-fish show progress in forms. 



The Catskill period, the top of the Devonian, shows more remains of 

 Selachian and Ganoid fishes. This period lasted long enough to lay 

 down strata to a depth of about 6,000 feet. The rocks of this period 

 have but few animal remains, and their species differ greatly from those 

 of the earlier periods. The plants are of the same character as those 

 of the Chemung. It thus appears that the fishes occur only in the mid- 

 dle and top formations of the great Devonian age in America, and that 

 there was an enormous duration of its time before any fish came to 

 America; that afterward they left, and another vast period elapsed be- 

 fore any more came. 



In the European Devonian, or "Old Red Sandstone," as the British 

 call it, the case is similar. In Scotland, the age is divided into seven 

 sub-divisions. The second from the bottom contains the Dipterus, Pter- 

 icftthys, Cocosteus, &c. ; the fourth contains the Cephalaspis, &c. , and 

 the top, the seventh, contains the ffoloptychius, &c. All those named 

 here belong to sub-divisions of the G-anoid fishes. This tribe is now 

 represented by the Polypterus, the Lepidosteus or Gar-fish, the Amia and 

 the Sturgeon. The Ganoids have a cartilaginous skeleton, instead of a 

 bony one as the Teleostei have. Their skull and lower jaw are some- 

 what bony, and there is a clavicle in the pectoral arch. The heart has 

 two lobes, an auricle and a ventricle. The spine extends to the end of 

 one branch of the tail, which is heterocercal or unequally divided. 



These fishes are, on the whole, inferior to the Teliosts or bony fishes, 

 which order comprises all the common fishes, such as the Eel, Herring, 

 Pike, Carp, Salmon, Trout, Cod, Perch, Mackerel, and many others. 



The skeleton of the Teliost is well ossified, including skull, jaw, clav- 

 icle, &c. His spine ends at the root of the tail fin, which is equally 

 divided or not divided at all. His optic nerves decussate or cross 

 each other. That he is later in time than the Ganoid, is proved 

 by embryology, since the embryo of the Teliost is the equivalent of the 

 adult Ganoid (See fig. 55). The Teliost does not occur in the Geological 

 strata till the Cretaceous times many long ages after the Devonian. 

 The Selachian or Placoid is inferior in many respects to either the Teli- 

 ost or the Ganoid. His skeleton is generally wholly cartilaginous. His 

 skull is a cartilaginous box without sutures. He has no clavicle, or 

 collar bone. His gills are not covered by a lid or operculuin as in other 

 fishes. His mouth is on the under side of his head. His heart is two- 

 lobed. To this order belong the Rays and Sharks. It is to some sub- 

 divisions of these two orders, Placoid or Selachian and Ganoid, that the 

 remains, found in the Devonian strata, belong. The Placoids are all of 

 three orders of the Shark tribes. While it is quite certain that the os- 



