The homocercal tail is the fan-shaped or symmetrical tail common among the Teleosts, or bony fishes. In its process of development the individual tail is first archicercal, then lophocercal, then diphycercal, then heterocercal, and lastly homocercal. A similar order is indicated by the sequence of fossil fishes in the rocks, although some forms of diphycercal tail may be produced by degeneration of the heterocercal tail, as suggested by Dr. Dollo and Dr. Boulenger, who divide diphycercal tails into primitive and secondary. The peculiar tapering tail of the cod, the vertebræ growing progressively smaller behind, is termed _isocercal_ by Professor Cope. This form differs little from diphycercal, except in its supposed derivation from the homocercal type. A similar form is seen in eels. [Illustration: FIG. 67.--Heterocercal tail of Young Trout, _Salmo fario_ (Linnæus). (After Parker and Haswell.)] The term _leptocercal_ has been suggested by Gaudry, 1883, for those tails in which the vertebral column ends in a point. We may, perhaps, use it for all such as are attenuate, ending in a long point or whip, as in the _Macrouridæ_, or grenadiers, the sting-rays, and in various degenerate members of almost every large group. The term _gephyrocercal_ is devised by Ryder for fishes in which the end of the vertebral axis is aborted in the adult, leaving the caudal elements to be inserted on the end of this axis, thus bridging over the interval between the vertical fins, as the name (~gephyros~, bridge; ~kerkos~, tail) is intended to indicate. Such a tail has been recognized in four genera only, _Mola_, _Ranzania_, _Fierasfer_, and _Echiodon_, the head-fishes and the pearl-fishes. [Illustration: FIG. 68.--Isocercal tail of Hake, _Merluccius productus_ (Ayres).] [Illustration: FIG. 69.--Homocercal tail of a Flounder, _Paralichthys californicus_.] The part of the body of the fish which lies behind the vent is known as the urosome. The urostyle is the name given to a modified bony structure, originally the end of the notochord, turned upward in most fishes. The term _opisthure_ is suggested by Ryder for the exserted tip of the vertebral column, which in some larvæ (_Lepisosteus_) and in some adult fishes (_Fistularia_, _Chimæra_) projects beyond the caudal fin. The urosome, or posterior part of the body, must be regarded as a product of evolution and specialization, its function being largely that of locomotion. In the theoretically primitive fish there is no urosome, the alimentary canal, as in the worm, beginning at one end of the body and terminating at the other. [Illustration: FIG. 70.--Gephyrocercal tail of _Mola mola_ (Linnæus). (After Ryder.)] =Homologies of the Pectoral Limb.=--Dr. Gill has made an elaborate attempt to work out the homologies of the bones of the pectoral limb.[4] From his thesis we take the following: "The following are assumed as premises that will be granted by all zootomists: