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.'l The term 

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

 lying horizontally, grow progressively smaller, as in Xeocera- 

 t'odiis, Protoptents, and other pipnoans and Crossopterygians. 

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

 genera Diplopterns and Gyropty chins, and for tails of this type it 

 should be reserved. 



The heterocercal tail is one in which the hindmost A-ertebra3 

 are bent upwards. The term is generally applied to those 



Fig. 64. Fig. 6.3 



Fig. 6-1. — Het-crocercul tail o[ Bowfin, Amia calva (Linna-us). (.Vt'ter Zittel.) 

 Fig. 6.5. — Heterocercal tail of Garpike, Le]>ifiosteus osscus (Linnseus). 



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 diphvcercal or 

 leptocercal by a process of degeneration, or in ordinarv fishes 

 becoming Jwmocercal. Dr. Ryder uses the teiTn heterocercal 

 for all cases in which any upbencling 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 througli 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. 



