340 PALiEOZOIC SYSTEM OF ROCKS. 



Hon, or Port Jackson shark of Australia (Fig. 442). This is charac- 

 terized not only by the strong bony spine supporting the median fins, 



/ but also by the cobble-stone-pavement teeth (Fig. 443) — characteristic 

 of Devonian Placoids. The family is called Cestracionts. The Devon- 



I ian Placoids were all Cestracionts. 



General Characteristics of Devonian Fishes. — Leaving out some 

 low aberrant forms, which are so soft and perishable that they are un- 

 likely to be preserved, and therefore are of little geological importance, 

 fishes may be conveniently divided into three orders — viz., Teleosts, 

 Ganoids,- and Placoids. Under Ganoids we include also the Dipnoi 

 (Lepidosiren and Ceratodus), because these, though quite distinct now, 

 run into each other completely in going backward in geological time : 

 (1.) Now, of these three orders, the Teleosts (perfect bone) are by far 

 the most numerous at present ; so much so, that the word fish calls up 

 at once this kind. Under this order come all ordinary or typical 

 fishes, such as the perch, the salmon, the cod, etc. In Devonian times, 

 on the contrary, there were no Teleost fishes. They were all Ganoids 

 and Placoids. Ganoids are now nearly extinct. 



(2.) Ganoids of the present day have some of them bony skele- 

 tons (Lepidosteus), but most of them cartilaginous skeletons. All the 

 Devonian Ganoids had cartilaginous skeletons. Placoids, both now and 

 in all times, had cartilaginous skeletons. Therefore, all Devonian 

 fishes, without exception, had cartilaginous skeletons. 



(3.) The position of the mouth of Teleosts is usually at the end of 

 the snout, or even often looking a little upward. Ganoids now, most 

 of them, have the mouth like Teleosts, at the end of the snout ; but 

 some (sturgeon) have it beneath on the ventral surface. The same 

 was true in Devonian times. The Lepido-ganoids had the mouth at 

 end of the snout ; but the Placo-ganoids usually on the ventral surface. 

 Placoids in all times have the mouth beneath. Therefore, all the De- 

 vonian fishes, except the Lepido-ganoids, or, we might say, the most 

 characteristic Devonian fishes, have the mouth in the ventral position. 

 4. The tail-fins of fishes are mainly of two t}'pes, the homocercal or 

 even-lobed (Fig. 444), and the heterocercal or uneven-lobed (Fig. 445). 

 The one is characteristic of Teleosts, the other of sharks and some 

 other fishes. These differ not only in shape, but still more in struct- 

 ure. In the former, the back-bone stops abruptly in a few large joints, 

 which send off the rays to the fin (Fig. 444, B), in the latter the back- 

 bone runs through the fin and gives off rays in pairs above and below 

 (Fig. 445, B). The former is a non-vertebrated, the latter a vertebrated, 

 tail-fin. There is still a third style in which the tail-fin is vertebrated 

 but not asymmetric as in Fig. 446, A and B. This style has been called 

 isocercal by Cope. It is probably the most primitive type. Now, in 

 living Ganoids, the tail-fin is vertebrated, though in some cases only 



