470 PROF. ST. GEORGE 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 Lepidosiren. In order to consider this point, however, 
I must recall to recollection conditions existing in the dorsal fin 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 Lepidosiren the azygos fins are supported by rays which are, though 
segmented, continuous with the neural and hemal 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 vertebre 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 vertebre. 
But, as I have before suggested, even if these spines of Ceratodus and Lepidosiren 
are neural spines, does that necessarily forbid their having been derived centripetally 
from the dorsal fin? 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 
1 L.c. p. 292. ? ZL. c. p. 293. 3 L. c. p. 292. 
