82 



Morphology of the Fins 



however, rudimentary basal rays are present, a condition differ- 

 ing from that of the early embryos. 



The diphycercal tail, as usually understood, is one in which 

 the end of the vertebral column bears "not only hypural but 

 also epural intermediary pieces which support rays." The term 

 is used for the primitive type of tail in which the vertebrae, 

 lying horizontally, grow progressively smaller, as in Neocera- 

 todus, Protopterus, and other Dipnoans and Crossopterygians. 

 The term was first applied by McCoy to the tails of the Dipnoan 

 genera Diplopterus and Gyroptychius, and for tails of this type it 

 should be reserved. 



The heterocercal tail is one in which the hindmost vertebras 

 are bent upwards. The term is generally applied to those 



FIG. 64. FIG. 65 



FIG. 64. Heterocercal tail of Bowfin, Amia calva (Linnaeus). (After Zittel.) 

 FIG. 65. Heterocercal tail of Garpike, Lepisosteus osseus (Linnaeus). 



fishes only in which this bending is considerable and is exter- 

 nally evident, as in the sharks and Ganoids. The character 

 disappears by degrees, changing sometimes to diphycercal or 

 leptocercal by a process of degeneration, or in ordinary fishes 

 becoming homocercal. Dr. Ryder uses *the term heterocercal 

 for all cases in which any upbending of the axis takes place, 

 even though it involves the modification of but a single ver- 

 tebra. With this definition, the tail of salmon, herring, and 

 even of most bony fishes would be considered heterocercal, and 

 most or all of these pass through a heterocercal stage in the 

 course of development. The term is, however, usually restricted 

 to those forms in which the curving of the axis is evident with- 

 out dissection. 



