FINS OP 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 PristiopJiorus 

 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. 

 Eeay 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 1 

 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 marinns, Mustelus canis, Galeocerdo tigrinus, Eulamia milierti, Sphyrcena 

 zygcena, Odontaspis litoralis, Acanthias americanus, Raia Iwvis, Myliohatis fremenmUei, 

 and Acipenser hrevirostris, as well as of some other structures, with a well-noted his- 

 torical summary of recent publications regarding the genesis of the cheiropterygium 

 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. 



