Fishes of the Western North Atlantic 5 



from its base as the fin rays grow in actinopterygian fishes,^* and presumably in coela- 

 canths. Although the radial cartilages that support the wing-like pectoral and pelvic 

 fins of skates and rays recall in appearance the jointed fin rays of bony fishes, they are 

 a part of the internal skeleton ; and they are single whereas the soft 

 rays of bony fishes are double (p. 3; Fig. i). 



Although the fin rays of the Dipnoi, which are intermediate 

 between elasmobranchs and actinopterygian bony fishes in this 

 respect, are jointed, they resemble the horny fin rays of elasmo- 

 branchs and chimaeroids in their growth inward from the outer edge 

 of the fin instead of outward from the base, in their arrangement in 

 two series, and in their seemingly single nature.'* 



The contrast between bony and cartilaginous fishes is further 

 blunted by the presence of hair-thin unjointed horny rays like those 

 of the elasmobranchs in the so-called adipose dorsal fin of certain 

 bony fishes (salmons and catfishes)" and in the free edges of the 

 rayed fins of other bony fishes." 



All of the other differences that have been proposed as alter- 

 native between bony and cartilaginous fishes are beset with excep- 

 tions. For example, it has been cited repeatedly that the dermal 

 fold, which roofs the common branchial cavity (into which the in- 

 ternal gill clefts open), is supported in bony fishes by a series of 

 opercular bones, whereas in the only living cartilaginous fishes that 

 have a corresponding dermal flap (chimaeroids) it is supported by a 

 series of cartilaginous rays only. The polyodontids (paddlefishes), 

 however, have only vestiges of the opercular bones; in the gigantu- 

 roids, in some stomiatoids, and in some eels they are ossified but 

 little; and in the lyomerids (an eel-like group of deep-sea fishes with 

 enormously distensible mouths) and in the lophobranchs they are lacking altogether.'* 



Below the gill covers, the great majority of bony fishes has a series of parallel 

 branchiostegal rays, but these rays are lacking in the elasmobranchs, chimaeroids, 

 Dipnoi, acipenseroids, and deep-sea Lyomeri. 



The scales of bony fishes are deeply implanted in the skin, are wholly of mesodermal 

 origin, and they usually persist throughout the life of the fish (for exceptions, see ftn. 4); 



Figure i. Soft dor- 

 sal fin ray of Esox 

 lucius (pike) with 

 its skeletal support; 

 LEFT-lateral view; 

 RicHT-front view. 

 R ray; p basal seg- 

 ment, and M median 

 segment of support- 

 ing radial. After 

 Goodrich, emended 

 somewhat. 



14. 



15' 



16. 



i7- 



18, 



For the development of soft rays (leptotrichia) in the actinopterygians, see especially Pennant (6j; 190-195) and 

 Bertin {31: 735). 



For the structure, growth, and homologies 'of the fin rays of the different major groups of living fishes, see espe- 

 cially Goodrich {2S: 480-491), Eaton (25: 195-200), and Bertin (jj: 731-736). 



According to Dr. George S. Myers (personal communication), what appear to be ossified spines are developed 

 in the adipose fin of some African catfishes, and with age in some characids of the genus Serrasalmus; also the lori- 

 cariid and calichthyid catfishes have the adipose fin supported by an initial bony spine. 



For a review of early literature on this subject with accounts and illustrations of these rays in the adipose fin of 

 salmon and trout, see Valette St. George {88: 187-192, pi. 15); for a more recent review, see Goodrich {28: 



473-477)- 



For a discussion of the evolution of the operculum in different groups of fishes, fossil and living, see Eaton 



[24: 42-46). 



