138 



shaped teeth, such as are now pos- 

 sessed by nearly all our sharks, com- 

 menced in small numbers. One of the 

 important dififerences between the 

 o^anoids and the teleosts or true fishes 

 is in the tail vertebrae. In the ganoids 

 the tail vertebrae decrease gradually in 

 size and curve upwards in the upper 

 lobe of the tail. In the teleosts the 

 tail vertebrae ends a short distance in 

 front of ends of the middle fin rays of 

 the tail fin. In the ganoids the upper 

 lobe of the tail fin is the largest. In 

 the teleosts both lobes are nearly the 

 same size. The tail of the ganoid fish 

 is called heterocercal, that of our mod- 

 ern or teleost fishes is homocercal. 

 The tail of all early ganoids was strong- 

 ly heterocercal. In the Triassic and 

 Jurassic its lobes in many cases became 

 nearly equal, approaching the homo- 

 cercal tail. The tails of all sharks are 

 heterocercal, of all modern fishes it is 

 homocercal except in a few families, 

 as the cod and related fishes, it is 

 Isocercal ; that is, the vertebrae de- 

 crease in size, but do not form an up- 

 ward curve. So far as we know the 

 Shad family is the first of our teleosts 

 or true fishes to appear, and these were 

 quite abundant iii the early part of the 

 Triassic. 



The rays, fish-like animals much 

 like Sharks, but with the body and fins 

 flattened or spread out in a broad flat 

 disc, appeared in the Jurassic. The 

 Chimeras, so abundant in the De- 

 vonian and which died out apparently 

 at the close of the Devonian, also re- 

 appeared at the beginning of the Juras- 

 sic. These did not belong to the same 

 families as did the more ancient 

 Chimeras. The Chimeras no doubt 

 flourished in the Carboniferous and 

 Triassic, but migrated to some portion 

 of the sea where now perhaps their re- 

 mains lie buried in rocks below the 

 bottom of the sea. Their survivors, 

 which were able to modify their struc- 

 ture and habits to become suited to 

 new conditions, returned in modified 

 forms in the Jurassic, where in time 

 their remains come to the surface as 

 fossils. 



At the end of the Cretaceous or be- 



ginning of the Tertiary we find all of 

 our modern types of sharks and all of 

 the important orders of teleosts. The 

 sturgeons and ganoids decreased 

 throughout the Tertiary or Quaternary 

 until at present we have but few living 

 species. The sturgeons are the more 

 abundant. Of the large group of 

 Ganoids so abundant during all these 

 geological ages but few forms are liv- 

 ing to-day. These are the Ceratodus, 

 lung fish of Australia ; the Polypterus 

 of the Nile, the Protopterus of West- 

 ern Africa, the Dogfish and the three 

 Garpike of North America. These few 

 species are but the remnants of a once 

 large and extensive group of fishes. 



In the study of fishes we notice that 

 some are highly specialized so far as 

 their structures are concerned ; theteeth 

 of some become especially fitted for a 

 peculiar kind of food, and as a result 

 quite unfit for any other kind. Some, 

 to be protected from their enemies, de- 

 velop a heavy armor, which only re- 

 tards their activity. Other fishes are 

 more generalized ; that is, are of me- 

 dium size, omnivorous habits, are not 

 hampered in their movements by a too 

 heavy coat of mail, etc. When any 

 change of conditions came to modify 

 their habits of living the specialized 

 were always the first to disappear. Be- 

 ing particularly fitted for one mode of 

 life made them all the more unfitted 

 for any other, and so when conditions 

 changed they perished. All of our 

 modern fishes except the few ganoids 

 are more or less specialized. The trout 

 lives in cool running water and some 

 varieties can live in no other, while 

 some fishes have become accus- 

 tomed to warm, stagnant water and 

 cannot live with the trout. What is 

 true in this respect of fishes is true of 

 land animals as well. The large, pon- 

 derous, slow-moving reptiles of the 

 Triassic, Jurassic and the Cretaceous, 

 and the large mammals of the Ter- 

 tiary and Quarternary could not exist 

 except under the peculiar conditions 

 of that time, and sooner or later had 

 to give way to the smaller, more active 

 and more resourceful animals of their 

 class. 



