l58 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. [l88^. 



any rate obvious that the protopterygian conditiou of the adipose fin is 

 a less specialized and more archaic one than that of its complete atrophy. 



The interval between the soft dorsal and the caudal is in part atro- 

 l^hied. The posterior part of this interval, however, gives rise to the 

 short accessory rays of the dorsal edge of the caudal. 



The truly dorsal part of the median fin-fold of the embryo salmon 

 ends at the notch m, towards w hich the urochord, later the urostyle, is 

 directed. At this point the proximal ends of the dorsal and ventral 

 actinotrichia embedded in the fold converge in a peuniform manner. 

 The actinotrichia below the notch n and extending out into the caudal 

 lobe cd give rise to the prolonged caudal rays, while those in the ventral 

 part of the fold just in front of the caudal lobe give lise to the inferior 

 accessory caudal rays. 



The fold in the interval between the caudal and anal a atrophies, to- 

 gether with its contained actinotrichia, while those in the widened anal 

 part of the fold a as far as the vent v give rise to the permanent rays 

 of the anal, into which the muscles of the fin grow at an early stage or 

 at about the same time that those of the dorsal are developed. 



The preanal part of the fold^« in front of the vent, together with its 

 contained actinotrichia, atrophies entirely during further growth and 

 development. 



It is thus shown by the development of the salmon that the most 

 primitive tj'pe of distribution of the vertical fins of osseous fishes was a 

 continuous one, because of the development of a continuous series of 

 actinotrichia or primitive rays, and that the forms which now exist and 

 most nearly realize tliis distribution of the rays of the vertical fins are 

 the Dipnoans, in which the rays are also scarcely more than well-devel- 

 oped actinotrichia, several of which, taken in succession, are homolo- 

 gous with a single ray of an adult Teleost. 



The Dipnoans present many other embryonic characters which seem 

 to be partially paralleled by what is transitory in the embryo salmon. 

 When we shall know more of the embryology of Ceratodus through the 

 efforts of Mr. Caldwell, who has succeeded in obtaining its ova, further 

 comparisons may be instituted between the ordinary Teleostean embryo 

 and that of the singularly specialized mud fishes.* 



Some of the oldest "Gauoids" had the vertebral axis persistently 

 chordal, unmodified anteriorly, and with an extended series of fins, in 

 these respects paralleling somewhat the condition which is transient in 

 the embryo salmon. These seem to me to be good reasons for accept- 

 ing Professor Cope's views as to the affiliation of certain of the Palaeo- 

 zoic fishes with the existing Isospondyli, in the absence, as the latter 



* It is important to examine the fin-folds of the tails of the larvae of such Amphibians 

 as Pseudis and Dactylethra to determine whether or not actinotrichia are present. 

 Should these earliest representatives of fin-rays be found in any of the Amphibia the 

 abrupt hiatus now existing between the Dipnoans and the latter would, in a great 

 measure, be bridged. 



