44 BULLETIN OP THE BUREAU OF FISHERIES. 



from the base of the spine. It is directed somewhat posteriorly and serves for the 

 attachment of a group of the deeper caudal muscles. The last two caudal fin vertebrae 

 are sharply modified. Ventral to the rudimentary centra there is a series of strong 

 and broad hypurals. In the king salmon there are five hypurals, the most anterior one 

 the strongest, and the individuals of the series diminishing in size toward the dorsal 

 lobe of the fin base. The development of the hypurals is commensurate with that of 

 the caudal musculature. 



Saddled over the ends of the hemal spines of the last two vertebrae of the caudal 

 peduncle, the spine of the first caudal vertebra, the hypurals, and the bony fusion of 

 neural spines previously described, are the series of paired fin rays constituting the 

 caudal fin. The fully developed rays are 19 in number, with about 12 rudimentary 

 rays above and as many below. The middle ray of the fully developed series represents 

 the axial ray. It is not only in the middle of the series but the interfilamenti caudales 

 muscles are inserted symmetrically with reference to this axial ray (fig. 4). These 

 rays form a joint of limited movement over the end of the skeletal complex to which 

 they are strongly anchored in a firm mass of ensheathing connective tissue. 



CAUDAL FIN MUSCLES. 



The muscles of the caudal fin are derived from the posterior myotomes of the 

 embryonic lateral muscles. The only probable exception is the interspinous muscle, 

 which is very intimately associated with the dermal fin rays and the skin itself. The 

 muscles are superficial and deep. 



SUPERFICIAL MUSCLES. 

 CAUDAL END OF THE MUSCULUS LATER.^LIS SUPERFICI.^LIS. 



This is a trunk muscle, but the details of its caudal insertion have been reserved for 

 description at this point. The lateralis superficialis or dark muscle is continued over 

 the lateral surface of the caudal peduncle to be inserted into the base of the tail. It 

 forms a sheath on each side of the mid-line of the caudal peduncle estimated in width 

 at two-thirds the distance from the mid-line of the peduncle to the dorsal and ventral 

 borders respectively. The muscle substance ceases posteriorly in the middle line at a 

 point directly under, i. e., ventral to the base of the first long dorsal caudal ray. The 

 caudal end of the muscle, i. e., marking the termination of the myomeres in its 

 tendon and fascias, is distinctively clavate. The dorsal myomeres are narrowed, and 

 the myocommata run together into a strong tendon that is attached to the bases of the 

 first, second, and usually the third long dorsal rays just exterior to and in the fascia 

 of the profundus lateralis. In a Baird specimen (small male) the last three dorsal myo- 

 meres are modified, the last two into a muscular slip running obliquely dorsalward and 

 caudalward to end in a dehcate flat tendon or fascia. The dorsal lobe of the superficialis 

 is rendered more prominent by the fact that the dorsal border of the muscle, just at the 

 base of the caudal peduncle, is attached to fascias which are intimately connected with 

 the myocommata between the ventral two-thirds and the dorsal third of the epaxial 

 half of the lateral muscle. There is considerable irregularity in the arrangement of 

 the muscular fibers of the last two or three myomeres of the dorsal lobes of this 

 muscle. A rather common irregularity is that shown in figure 5. The ventral lobe of 



