138 



shaped teeth, such as are now pos- ginning of the Tertiary we find all of 



sessed by nearly all our sharks, com- our modern types of sharks and all of 



menced in small numbers. One of the the important orders of teleosts. The 



important differences between the sturgeons and ganoids decreased 



ganoids and the teleosts or true fishes .throughout the Tertiary or Quaternary 



is in the tail vertebrae. In the ganoids until at present we have but few living 



the tail vertebrae decrease gradually in species. The sturgeons are the more 



size and curve upwards in the upper abundant. Of the large group of 



lobe of the tail. In the teleosts the Ganoids so abundant during all these 



tail vertebrae ends a short distance in geological ages but few forms are liv- 



front of ends of the middle fin rays of ing to-day. These are the Ceratodus, 



the'tail fin. In the ganoids the upper lung fish of Australia; the Polypterus 



lobe of the tail fin is the largest. In of the Nile, the Protopterus of West- 



the teleosts both lobes are nearly the ern Africa, the Dogfish and the three 



same size. The tail of the ganoid fish Garpike of North America. These few 



is called heterocercal, that of our mod- species are but the remnants of a once 



ern or teleost fishes is homocercal. large and extensive group of fishes. 



The tail of all early ganoids was strong- In the study of fishes we notice that 



ly heterocercal. In the Triassic and some are highly specialized so far as 



Jurassic its lobes in many cases became their structures are concerned ; theteeth 



nearly equal, approaching the homo- of some become especially fitted for a 



cereal tail. The tails of all sharks are peculiar kind of food, and as a result 



heterocercal, of all modern fishes it is quite unfit for any other kind. Some, 



homocercal except in a few families, to be protected from their enemies, de- 



as the cod and related fishes, it is velop a heavy armor, which only re- 



Isocercal ; that is, the vertebrae de- tards their activity. Other fishes are 



crease in size, but do not form an up- more generalized ; that is, are of me- 



ward curve. So far as we know the dium size, omnivorous habits, are not 



Shad family is the first of our teleosts hampered in their movements by a too 



or true fishes to appear, and these were heavy coat of mail, etc. When any 



quite abundant in the early part of the change of condition's came to modify 



Triassic. their habits of living the specialized 



The rays, fish-like animals much were always the first to disappear. Be 

 like Sharks, but with the body and fins ing particularly fitted for one mode of 

 flattened or spread out in a broad flat life made them all the more unfitted 

 disc, appeared in the Jurassic. The for any other, and so when conditions 

 Chimeras, so abundant in the De- changed they perished. All of our 

 vonian and which died out apparently modern fishes except the few ganoids 

 at the close of the Devonian, also re- are more or less specialized. The trout 

 appeared at the beginning of the Juras- lives in cool running water and some 

 sic. These did not belong to the same varieties can live in no other, while 

 families as did the more ancient some fishes have become accus- 

 Chimeras. The Chimeras no doubt tomed to warm, stagnant water and 

 flourished in the Carboniferous and cannot live with the trout. What is 

 Triassic, but migrated to some portion true in this respect of fishes is true of 

 of the sea where now perhaps their re- land animals as well. The large, pon- 

 mains lie buried in rocks below the derous, slow-moving reptiles of the 

 bottom of the sea. Their survivors, Triassic, Jurassic and the Cretaceous, 

 which were able to modify their struc- and the large mammals of the Ter- 

 ture and habits to become suited to tiary and Quarternary could not exist 

 new conditions, returned in modified except under the peculiar conditions 

 forms in the Jurassic, where in time of that time, and sooner or later had 

 their remains come to the surface as to give way to the smaller, more active 

 fossils. and more resourceful animals of their 



At the end of the. Cretaceous or be- class. 



