11 



Pleiironectidse has been produced by the action of natural 

 selection, i.e., by the aecunmlated 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 occiput 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 {O.C., P.C.), and of the latter, the 



eyeless one, as may be assiimed from the description of the 



atlas, is larger than the ocular. In those two extensions 



of the auditory capsule, the epiotic and parotic processes 



t Cp. especially, Traquair, Trans. Liuu. Soc, xxv., p. 263. Space 

 forbids a discussion of the literature in the text, but the following papers 

 should be consulted : — Schmid-Moimard, Jena. Zeits., xxxix. ; T. J. Parker, 

 Trans. Zool. Soc, xii., p. 5 ; Sagemehl, Morph. Jahrb., ix., p. 177 ; x., p. 1 ; 

 xvii., p. 489; Shufeldt, Eeport 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, Trans. Lirm. Soc, ser. ii. vii., p. 115; W. K. 

 Parker, Phil. Trans., vol. 173, pp. 139 and 443 : vol. 163, p. 95 ; McMurrich, 

 Proc Canadian Inst., N. S., ii., p. 270; Brooks, Sci. Proc R. Soc, Dublin, 

 K.S., iv., p. 166. 



