11 
Pleuronectidx has been produced by the action of natural 
selection, 2.e., by the accumulated effects of congenital 
variations. 
B.—THE SKELETON. 
This may be divided, as usual, into an axial and an 
appendicular skeleton. | We shall describe the former 
first, but the precise order must to a certain extent depend 
on convenience rather than upon strict logical sequence. 
We therefore begin with the cranium itself, afterwards 
proceeding to the remaining constituents of the skull, 
then to the vertebral column and unpaired fins, and finally 
to the limb girdles and paired fins. 
1.—Craniumt (Figs. 1 to 4). 
Owing to the difficulty of making an independent 
preparation of the chondrocranium it is, in the following 
description, described in the pieces into which it is divided 
when the cranium is disarticulated. 
Seen from behind (Pl. II., fig. 4) the ceciput is 
markedly asymmetrical, and a line traversing median 
structures would be convex towards the ocular side. This 
is observable also in the occipital condyle and in the 
paroccipital condyles (0.C., P.C.), and of the latter, the 
eyeless one, as may be assumed from the description of the 
atlas, is larger than the ocular. In those two extensions 
of the auditory capsule, the epiotic and parotic processes 
+ Cp. especially, Traqwair, Trans. Linn, Soc., xxv., p. 263. Space 
forbids a discussion of the literature in the text, but the following papers 
should be consulted :—Schmid-Monnard, Jena. Zeits., xxxix.; T. J. Parker, 
Trans. Zool. Soc., xii., p. 5; Sagemehl, Morph. Jahrb., ix.,p.177; x., p.1; 
Xvli., p. 489; Shufeldt, Report U. S. F. C., 1883, p. 747; Allis, Jour. 
Morph., xii., p. 487; xiv., p. 425; Zool. Bull.,i., p. 1; Anat. Anz.,xvi., 
p. 49; xvii., p. 433; Cole, Trams. Linn. Soc., ser. ii. vii., p. 115; W. K. 
Parker, Phil. Trans., vol. 173, pp. 189 and 443: vol. 163, p.95; McMurrich, 
Proc, Canadian Inst., N. S., ii., p. 270; Brooks, Sci. Proc. R. Soc., Dubline 
INS S:, tv., Ds 166; 
