508 William k. Gregory 



Stage i, Diphycercy Notochord straight, opisthure symmetrical and 

 separate from caudal fin. 

 Examples: Amphioxus, Pakeospond-ylus, Cyclos- 

 tomes, certain Chimagroids (Hariotta), certain 

 Acanthodians, Chlamydoselachus, embryonic 

 sharks, ganoids, and teleosts. 

 Stage 2, Heterocercy Derived from diphycercy by development of the 

 true caudal fin beneath the notochord, the 

 opisthure upturned. The dermal portion may 

 become f an-like (rhipidoid) . 

 Stage 3, Homocercy Derived from heterocercy by progressive upturning 

 and reduction of the opisthure and by the co- 

 alescence of the posterior interhasmals into 

 broad "hypurals," which form a fan-shaped 

 (rhipidoid) bony tail. The dermal portion may 

 be fan-like (rhipidoid) or pointed (gephyroid). 

 Examples: Clupea, Salmo. 

 Stage 4, Euhomocercy Derived from homocercy by the loss of the 

 (New term) opisthure, the reduction of the hypurals and 

 epurals, which are functionally replaced by 

 ossified dermal rays, producing a perfectly 

 symmetrical bony tail. 

 Examples: mackerel group. 

 ( = Homocercal rhipidocercy Dollo.) 

 Stage 2', Gephyrocercy Derived from heterocercy by degeneration of 

 the opisthure; the posterior portion of the 

 median superior and inferior fins fuse posteriorly 

 to form a new and symmetrical, pointed tail fin. 

 Examples: Polypterus, Ceratodus. 

 ( = Heterocercal gephyrocercy Dollo.) 

 Stage 3', Hypocercy (new term). Derived from homocercy by co- 

 alescence of the fan-like caudal with the pro- 

 longed anal, the conjoined inferior fins being 

 then pulled out into a long pointed tail fin. 

 Example: Notopterus, Macrouridas. 

 (=Homocercal gephyrocercy Dollo.) 

 Stage 3' Isocercy Derived from homocercy or hypocercy by the 

 or 3," atrophy of the pointed tail and the development 



of a new fan-shaped tail around the stump of 

 the old one. 

 Examples: Gadus, Anguilla Simenchelys. 

 ( =Dorso-caiidal rhipidocercy Dollo.) 

 Leptocercy A condition in which the tail ends in a long delicate 

 wisp, often an adaptation to deep-sea conditions. 

 May be derived from Stages 1, 2, 2', or 3'. 



