416 OSBURN 



fin, deriving the median fin skeleton from processes (dorsal and 

 haemal spines) of the axial skeleton, while according to the 

 fin-fold theory the skeleton is supposed to have been developed 

 independently of the vertebral column. But it is in the origin 

 of the paired fins that differences of opinion are most in evidence, 

 for while the fin-fold theorists consider that the paired fins have 

 had in a general way a similar origin to the unpaired, the gill- 

 arch theorists hold that they have been modified from gills, 

 and that the girdles and rays of the fins are directly homologous 

 with the supporting structures (arches and rays) of the gills. 



During all the years in which these theories have been under 

 discussion unsettled points have not been lacking, and within 

 recent time a number of objections have been urged against the 

 fin-fold theory. With the hope of deciding some of these vexed 

 questions and with a view to testing the validity of the objec- 

 tions raised, the writer has been led to investigate the develop- 

 ment of the paired and unpaired fins of a cestraciont shark 

 (Heterodontus japonicus, Dumeril), a form belonging by direct 

 lineage to a group of very ancient sharks, — much older, as far 

 at least as can be judged by palasontological data, than any 

 selachian that has hitherto been investigated. 1 For comparison 

 the writer has had various stages of Spinax, Mustelus, and 

 Torpedo, and has been especially fortunate in having access to 

 a number of embryonic stages of Chlamydoselachus anguineus 

 Garman, a form generally recognized as being one of the most 

 primitive of modern Selachii. The 'series of Cestracion (Hetero- 

 dontus) embryos at my command is very complete. For the use 

 of all this valuable material my thanks are due to Professor 

 Bashford Dean, to whom I am also grateful for much encourage- 

 ment and many helpful suggestions in pursuit of the work. 



The present paper embodies only the main results of my 

 studies and will be followed by another more extended in scope, 

 in which will be given the evidence upon which these results 

 are based and in which the literature of the subject will be treated. 



It may be briefly stated that the results of my work indicate 

 that many of the objections raised against the fin-fold origin 



i All references to Cestracion, Chlamydoselachus, and Spinax in the 

 following pages are from my own observations, unless otherwise accredited. 



