FOSSIL VERTEBRATES— FISHES 399 



spinal column. In the original condition the dorsal and the 

 ventral parts of the fin were equally developed, and the ver- 

 tebral axis divided the fin into two equal parts, so that the fin 

 presented a rounded or slightly acuminate appearance. This is 

 called the diphycercal condition. Another, and a very common 

 condition among the more primitive forms of fish, is where the 

 ventral portion of the caudal fin is developed at the expense of 

 the dorsal, and the terminal portion of the vertebral axis is bent 

 upward at the end. This is called the heterocercal condition. 

 The form most commonly found in the modern forms of fishes 

 is that in which both the dorsal and the ventral portions of the 

 caudal fin are developed more than the main portion of the fin, 

 but about equally themselves. This is called the homocercal 

 condition, and was for a long time considered as the primitive 

 condition, or at least more primitive than the heterocercal. It 

 is shown to be untrue by the fact that even in the highest types 

 of the bony fishes the extremity of the notochord is bent upward 

 as in the heterocercal form of the tail. The progress of devel- 

 opment of the tail seems to have been from the diphycercal 

 through the heterocercal to the homocercal. 1 



The oldest remains of fishes that are definitely known come 

 from the Upper Silurian and the succeeding rocks as high as 

 the Carboniferous. The group called Ostracodermi has long been 

 denied a position among the fishes by certain authors, the prin- 

 cipal objection being the seeming lack of any lower jaw, which 

 is regarded as one of the principal characters of the vertebrates. 

 Because of this feature and the appearance of some of the forms 

 which is similar in a general way to that of the Trilobites, the 

 group has been considered as belonging to the Crustacea, but 

 there are so many other characters that unite them with the 



1 A discussion of the facts here pointed out, with much more that is valuable to 

 the student, will be found in a recent book by Bashford Dean, Fishes, Living and 

 Fossil, an outline of their forms and probable relationships, Columbia University Bio- 

 logical Series, Macmillan & Co., 1895. This book takes up the various forms of fishes 

 in a popular way that will not be beyond the student of geology who has the princi- 

 ples of biology. The large number of excellent illustrations will be found to be of 

 great value in getting an idea of the fossil forms. 



