1890.] Lhe Homologies of the Fins of Fishes. 421 
(chevron) axonost and baseost. Such appears to have been the 
case with the Dipnoi, but whether it is strictly the case in the 
primitive Xenacanthus remains to be ascertained. The axonost 
and baseost are lost from the modern types of diphycercal tails, 
as Anguilla (Ryder, Pl. 1v., Fig. 4) and Gadus. 
The change from diphycercy to heterocercy is seen in the 
upward curvature of the extremity of the vertebral column, and 
the development of a portion of the inferior fin (anal) into a 
posteriorly projecting angle. This is the state of affairs in the 
Elasmobranchii, Chondrostei, and various extinct Teleostomata. 
The change from heterocercy to homocercy is seen in the in- 
creased recurvature of the column, and the successive abortion 
of its extremity. This is accompanied by the increase in antero- 
posterior diameter of the hypural bones, especially distally, since 
they develop to occupy the space gained by ‘the recurvature. 
The neural spines corresponding to them become correspondingly 
reduced. Such modified elements have been called (provisionally) 
hypural bones.. All stages of development of these bones 
may be formed in the Teleostomata. Thus in the lower 
forms (most Malacopterygia) they remain distinct from each 
other. In many of the Acanthopterygia—e. g., Cottus, (Pl. xviIL., 
Fig. 1,) and Pharyngognathi, they are fused together, forming a 
continuous fan-shaped body, which supports the fin-rays directly. 
The different types of hypural bones are well illustrated in the 
accompanying plates from Ryder. 
EXPLANATION OF THE PLATES. 
? 
For the Plates XV. to XVIII., which are taken from those illustrating Pro- 
fessor J. A. Ryder’s memoir on “ The Origin of Heterocercy,” I am indebted 
to the Hon. Marshall MacDonald, U. S. Commissioner of Fish and Fisheries. 
PLATE XIV.—Restoration of Xenacanthus decheni, an Ichthyotomous 
Elasmobranch from the Coal Measures of Alsace, much reduced. From 
Brongniart and Sauvage. 
PLATE XV.—Fig. 1, Chimera monstrosa L. &. Fig. reduced from 
Agassiz’s Poissons Fossiles, showing opisthural filament. Fig. 2, Side view 
