FINS OF ELASMOBRANCHS. 467 
explain the condition of these parts in Ceratodus and Lepidosiren, and would lead us 
to regard the spinous processes of the human skeleton as essentially or originally 
exoskeletal structures (dorsal radials) which have adhered to and grown to be connate 
with the subjacent axial skeleton. Moreover it seems almost impossible not to regard 
at least the distal parts of the basal plates of the dorsal fin in Pristis and Pristiophorus 
as answering to the basal cartilage of the dorsal of Notidanus. 
But if the dorsal and anal fins have been developed centripetally, how about the 
paired fins? Did the hard parts developed within them shoot forth from the skeletal 
axis, or grow inwards towards that axis? 
Now, in the first place, the reasons which have just been enumerated why the 
paired fins cannot have an antero-posteriorly longitudinal attachment to the skeletal 
axis are equally reasons against the growing forth from that axis of the hard parts 
formed within them. 
Again, if we look at the ventral fins, and consider the multiplicity of their radial 
parts, together with the simplicity of their support, it seems incredible that the former 
should have been formed by a centrifugal chondrification. But if the centripetal 
process be conceded as that by which the paired fins were formed, it reflects additional 
probability on the centripetal formation of the azygos fins also. 
My examinations and reflections had proceeded thus far when my friend Prof. J. 
Reay Greene called my attention to a paper on “median and paired fins,” noticed in 
Silliman’s Journal as having been published in the third volume of the ‘ Transactions’ 
of the Connecticut Academy. 
Only the first part of the third volume was to be obtained at any scientific library 
known to me in London; and the ‘Transactions’ of this Academy do not seem to find 
their way to our National Library at the British Museum. Under these circumstances 
I addressed myself directly to the author, Mr. James K. Thacher, who, with extreme 
kindness, sent me from America a separate copy of his valuable paper. 
The paper was to me of the highest interest. I found by it that its author and I 
had been simultaneously following out to generally similar results a similar line of 
thought, though in one important point he had gone beyond me. 
The memoir contains good figures and descriptions of the dorsal-fin cartilages of 
Petromyzon marinus, Mustelus canis, Galeocerdo tigrinus, Eulamia milberti, Sphyrena 
zygena, Odontaspis litoralis, Acanthias americanus, Raia levis, Myliobatis fremenvillei, 
and Acipenser brevirostris, as well as of some other structures, with a well-noted his- 
torical summary of recent publications regarding the genesis of the chetropterygium 
and a reasoned statement of his own views as to the nature and homologies of verte- 
brate limbs. 
Singularly enough, the author states that ‘‘ the origin of his paper lay in an observa- 
tion of a fin of Raia” —apparently the very resemblance between the dorsal and ventral 
fin-structure, which so greatly and so early impressed me. 
