STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 805 



Huxley 1 in his paper on the development of the caudal fin of 

 the Stickleback, holds that these elements are of the nature of 

 interhaemal bones. He says (p. 39) : " The last of these rings lay 

 just where the notochord began to bend up. It was slightly 

 longer than the bony ring which preceded it, and instead of 

 having its posterior margin parallel with the anterior, it sloped 

 from above downwards and backwards. Two short osseous 

 plates, attached to the anterior part of the inferior surface of the 

 penultimate ring, or rudimentary vertebral centrum, passed down- 

 wards and a little backwards, and abutted against a slender 

 elongated mass of cartilage. Similar cartilaginous bodies occupy 

 the same relation to corresponding plates of bone in the anterior 

 vertebrae in the region of the anal fin ; and it is here seen, that 

 while the bony plates coalesce and form the inferior arches of 

 the caudal vertebrae, the cartilaginous elements at their ex- 

 tremities become the interhaemal bones. The cartilage connected 

 with the inferior arch of the penultimate centrum is therefore an 

 " interhaemal " cartilage. The anterior part of the inferior surface 

 of the terminal ossification likewise has its osseous inferior arch, 

 but the direction of this is nearly vertical, and though it is con- 

 nected below with an element which corresponds in position 

 with the interhaemal cartilage, this cartilage is five or six times 

 as large, and constitutes a broad vertical plate, longer than it is 

 deep, and having its longest axis inclined downwards and back- 

 wards. . . . 



" Immediately behind and above this anterior hypural apo- 

 physis (as it may be termed) is another very much smaller vertical 

 cartilaginous plate, which may be called the posterior hypural 

 apophysis." 



We have seen that Mivart expresses himself doubtful on the 

 subject. Gegenbaur 2 appears to regard them as haemal arches. 



The latter view appears to us without doubt the correct one. 

 An examination of the tail of normal Teleostei shews that the 

 fin-rays of that part of the caudal fin which is derived from the 

 ventral lobe of the larva are supported by elements serially 

 homologous with the haemal arches, but in no way homologous 



1 "Observations on the Development of some parts of the Skeleton of Fishes," 

 Quart. Journ. Micr. Science, Vol. VII., 1859. 



2 Elements of Comparative Anatomy. (Translation), p. 431. 



