470 PROF. ST. GEOEGE MIVART ON THE 



Mr. Thacher does not share my views as to the nature of the solid supports of the 

 azygos fins in Ceratodus and Lepidosireti. In order to consider this point, however, 

 I must recall to recollection conditions existing in the doi'sal fiu of Pristis, Pristio- 

 phorus, and Notidanus. As to the first two genera, it seems to me, as I have before 

 said, impossible not to regard their continuous ray-bearing cartilages as homologous with 

 the continuous ray-bearing cartilage of Notidanus. The study of development will 

 show how they arise in the individual, but not necessarily how they arose in the 

 race ; for having once acquired union with the axial skeleton centripetally, they may 

 subsequently have come to arise in continuity with that skeleton. The fact that in 

 Ceratodus and Lepidosireti the azygos fins are supported by rays which are, though 

 segmented, continuous with the neural and haemal spines, does not, to my mind, 

 necessarily prove them to be of axial origin. Mr. Thacher, as I have said, thinks 

 otherwise; he says^: — "The cartilaginous supports of the median fold of the Dip- 

 noans are very long and segmented ; they are simply elongated neural spines, and not 

 primordial fin-rays in any homological sense. If they were formed by the reduction 

 in number of the primordial fin-rays and their coalescence with the neural spines, it 

 is impossible that we should not have here and there an extra one, and some evidence 

 in the case of others of such a junction." He is even disposed ^ to associate 

 the Dipnoi with the Amphibia and Amniota as one great generic group, on the 

 strength of their having entirely lost the '• primordial median fin-rays." If they were 

 really so lost, it would be indeed an important character : but I do not see the force 

 of his argument in favour of their having been so ; for since the median fin-rays have 

 become, in some Elasmobranchs, equal in number to the vertebrae for a varying extent 

 of the vertebral column, what is there improbable in that correspondence having 

 become, in a few exceptional forms, complete] Again, the Dipnoi must at least be 

 allowed to be allied to the Ganoids ; and in Lepidosteus and Amia we find a near, 

 though not an accurate, correspondence in number between the dorsal-fin radials and 

 the subjacent vertebrse. 



But, as I have before suggested, even if these spines of Ceratodus and Lepidosiren 

 are neural spines, does that necessarily forbid their ha\ing been derived centripetally 

 from the dorsal fin 1 It is possible that all neurapophyses may have been so derived. 

 As to this question, however, the study of development can alone decide. At any rate 

 undoubted neural spines may retrogressively assume the condition of dorsal radials, as 

 in those lizards (e. g. Basiliscus) in which these processes send off delicate prolongations 

 into the long tegumentary processes of the dorsal crest. 



Mr. Thacher, in controverting the view here advocated, and affirming ^ that " neither 

 are median fin-rays derived from neural spines, nor neural spines from fin-rays," brings 

 forward as an argument the condition of these parts in Acipenser as showing the 

 distinctness between the neurapophysial and fin-ray elements. But this very instance 



' L. c. p. 292. » L. c. p. 293. = L. c. p. 292. 



