398 PECULIARITIES IN THE STRUCTURE 



I found that the rays of the tail-fin, and their interspinous 

 bones, were crowded together in a direction from behind for- 

 wards, and abutted against the superior spinous process of the 

 fourteenth and the inferior spinous process of the fifteenth 

 vertebra. The sixteenth, seventeenth, and eighteenth vertebrae 

 assumed the appearance, the two former combined, of an 

 interspinous bone, and the latter of a fin-ray, and could not 

 have been distinguished from these but by their direct con- 

 tinuation with the bodies of the vertebrae, and their more 

 cylindrical and shorter form. The joint between the seven- 

 teenth and eighteenth was in the line of the articulations of 

 the fin-rays and their interspinous bones, the ultimate vertebra 

 assuming the appearance and function of a fin-ray, the 

 penultimate and antepenultimate combined of an interspinous 

 bone. 



The interest involved in this form of skeleton consists in 

 the explanation it affords of the true nature of the so-called 

 last vertebra in the spinal column of fishes. Is that fan-shaped 

 bone a vertebra ? or is it a composite bone, containing the 

 elements of a number of vertebrae and of interspinous bones 

 of fiii-rays ? I have always been led to conclude that it is a 

 composite bone, and it required only such an arrangement of 

 skeleton as that now under consideration to afford a natural 

 analysis of the tail in this class of fishes, and to prove the 

 correctness of the opinion to which I have just alluded. In 

 many of the osseous fishes the last bone of the spine exhibits 

 traces of a central element, and in some families (Taenioidas) 

 it appears to be prolonged far beyond the caudal fin, in the 

 form of a fine filament, but in none, as far as I am aware, is it 

 arranged as in the present instance. 



The next peculiarity is in the muscular system. The sun- 

 fish exhibits not a trace of abdominal muscles.* The viscera 

 from the spine to the median line of the belly are inclosed by 



* Meckel, Comparative Anatomy, torn. v. p. 185. 



