1876.] PROF. T. H. HUXLEY ON CERATODU8 FORSTERI. 49 



and the skeletal elements on the dorsal side of the axis of the fish-fia 

 answer to those on the ventral side of the axis in the amphibian limb. 



As Dr. Giinther has observed, the contour of the fin in Ceratodua 

 is somewhat like that of a sickle. The praeaxial edge is convex 

 forwards and rather thicker, especially at its proximal end, than 

 the posterior edge, which is concave backwards in its distal and 

 convex in its proximal half. The apex of the fin is slender and re- 

 curved. A rounded and narrowed neck unites the limb with the 

 trunk. 



Thus the limb, as a whole, is essentially unsymmetrical when its 

 postaxial and prseaxial halves are compared. A corresponding asym- 

 metry is strikingly obvious in the skeleton when it is prepared by 

 removing the integument and muscles of the dorsal face, while the 

 undisturbed condition of the parts is preserved by leaving the ventral 

 integument and muscles untouched (fig. 10). 



It will be seen that, on the praeaxial side (Pr.a), each of the sub- 

 quadrate segments of the median part of the skeleton, except the first 

 and the terminal segments, gives attachment by its distal angle to a 

 single jointed ray. The proximal or first ray (R) is much stouter than 

 any of those which succeed it; and all take a direction approximately 

 parallel to one another, their long axes forming an acute angle 

 with that of the series of median segments. In the distal portion 

 of the fin, the postaxial rays have a similar arrangement, and are 

 only more slender than the prseaxial rays. But the second seg- 

 ment bears no fewer than five rajs. Of these, the proximal, which 

 is shortest and slenderest, stands out at right angles to the axis of 

 the series of median segments ; while the others are gradually in- 

 clined at a less and less angle to it. The third segment and the 

 fourth each carry two postaxial rays ; the rest have but one. 



Dr. Giinther's figures show that, in his specimen also, the fourth 

 and the third segments each bore two postaxial rays ; but there are 

 only four attached to the second segment, and all these are represented 

 as if they had nearly the same inclination to the axis of the fin as the 

 praeaxial rays. 



To dwell so strongly upon these minutioe may seem to be making 

 a great deal of a very small matter ; but its importance becomes 

 manifest when the fin of Ceratodus is compared with that of other 

 fishes. 



In my " Preliminary Essay on the systematic arrangement of tha 

 Fishes of the Devonian epoch"*, I made use of the term "Crosso- 

 pterygian" to express a peculiarity which is very strikingly manifest 

 in the fishes to which I applied it, the fin-rays of the paired'fins being 

 disposed, like a fringe, round an oval, or elongated, central space 

 covered with scales. The Crossopterygii, however, were not defined 

 by this character alone ; and hence the fact that truly fringed fins are 

 found beyond the limits of that group does not interfere with its per- 

 fectly natural character. In strictness, all fishes which possess paired 

 fins are Crossopterygian in so far as the fin-rays always fringe the 



• Memoirs of the Geological Surrey of the United Kingdom, decade x. 1861. 

 Proc. Zool. Soc— 1876, No. IV. 4 



