GENERAL OBSERVATIONS ON FINS. SJ 



point. One or two extraordinary departures from the 

 ordinary form of this fin may here be noticed, but they 

 are mostly confined to single genera. In Trachypterus , 

 the fin, although large and truncate, is mounted vertically 

 upon the point of the tail, so as to form an angle with 

 the line of the body. This structure is altogether unique 

 among fishes ; for it does not exist in the neighbouring 

 genera Argycthis Sw. and Nemotherus Raf., whose tails 

 are situated as in ordinary fishes. The other modifi- 

 cation belongs to Cuvier's Serranus phcBton {fig. 4. e): 

 the tail is forked; but from the centre or deepest part 

 of the cleft springs a long filamentous ray, near three 

 times the length of the fin itself, — a structure of which, 

 as yet, we know of no parallel. 



(38.) Having now brought before the reader (what 

 has never hitherto been done) an enumeration of nearly 

 all the different forms observable in the fins of fishes, 

 we shall conclude this part of our subject with an 

 attempt to generalise, in some degree, the facts thus 

 brought together, in order to show that the results thus 

 obtained will correspond in some remarkable points 

 with the locomotive organs of birds. In the first place, 

 it must be remembered that these organs are more nu- 

 merous in fishes than in any other vertebra ted animals . 

 this is the necessary consequence of their being the 

 fiissirostral or aquatic type of the vertebrated circle ; 

 which type, as we formerly explained, invariably possesses, 

 in this circle, the greatest powers of motion. The 

 ornithologist is quite aware of this ; but it may be as well 

 to inform the ichthyologist, who may not have studied 

 that branch of zoology, that the swallow, goatsucker, 

 tern, albatross, and kite, — the swiftest flying birds that 

 are known to exist, — are all of them of the fissirostral 

 structure, whether by affinity or analogy : and thus do 

 we find this law pervading the class before us, — a class 

 which may be said to be in perpetual motion ; for 

 although a quadruped can lay down to repose, and a 

 bird can roost on its legs, it seems difficult to imagine 



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