THE ORIGIN OF VERTEBRATE LIMBS 433 



similar nature and potentiality of the two kinds of fins at a 

 very remote period, — Upper Silurian and Devonian. 



X. Horny fin-rays or ceratotrichia are very characteristic 

 structures of the fins of sharks and occur nowhere else in the 

 body. The importance of these structures in phylogeny has 

 recently been discussed by Goodrich ('04) who finds them to be 

 very ancient and very conservative. They occur alike in the 

 paired and unpaired fins of all sharks, even the most ancient. 

 (Goodrich's failure to find the ceratotrichia in the paired fins 

 of Cladoselache can be attributed only to insufficient material, 

 for an examination of the many specimens in the American 

 Museum of Natural History proves their presence beyond a 

 doubt.) As these structures occur equally in every respect in 

 all the fins, and develop in the same way and at the same relative 

 time, they may be taken to indicate a community of origin for 

 all of the fins. 



When we consider the facts derived from embryology, anat- 

 omy, and palaeontology which are arrayed in the preceding 

 pages, the conclusion is borne in upon us that the paired and 

 unpaired fins are primarily similar structures, and the evidence 

 from the present investigations is overwhelmingly in favor of 

 the origin of all fins as local outgrowths from the body wall. 



Columbia University, New York City, 

 March 28, 1906. 



LITERATURE REFERRED TO. 



Balfour, F. M. , 



'78. Monograph on the Development of Elasmobranch 

 Fishes. London, 1878. 

 Brauss, H. 



'98. Ueber die Extremitaten der Selachier, Verh. Anat. 



Gesellschaft. Apr., 1898. 

 '04. Tatsachliches aus der Entwickelung des Extre- 

 mitatenskelettes bei den niedersten Formen, Hackel- 

 festschrift (Jenaischer Denkschriften XI), Jena, 1904. 

 Bunge, A. 



'74. Ueber die Nachweisbarkeit eines biserialen Archi- 

 pterygium bei Selachiern und Dipnoern. Jena. 

 Zeitschr. fur Naturwissenschaft, Bd. VIII, Jena 

 1874. 



