282 ./. K. Tharher — MedUm and Paired Fins. 



Median Fins in Myxina. 



In Myxine 'glutinosa the median tin extends but a short distance 

 forward. In a specimen 24 centimeters long, from the Bay of Fundy, 

 the fin reaches 4 cm. from the extremity of the tail on the dorsal side, 

 and 2*5 on the ventral. 



The fin-rays, now unquestionable homolognes of the ])rimordial fin- 

 rays of Gnathostomes, tliough not yet having assumed the histologi- 

 cal structure of true cartilage, support the thin fold of skin which 

 forms the fin. They are simple tapering rods, extending distally to 

 the edge of the fin, and proximally scarcely dipping below the general 

 body contours. 



The only deviation from sim])le rods which I have been able to find 

 is the dichotomous splitting of some of the rods where the fin rounds 

 the extremity of the tail. 



The numerical relation between these rays and the corres})onding 

 muscular segments is as three to one on the dorsal side, and as two 

 and a half to one on the ventral. 



I have been iinable to detect any muscular fibers in the composi- 

 tion of the fin. 



Median Fins in Petromyzon. 



Here the median fins are much better developed. In a specimen 

 (^Petromyzon marinus, from the Connecticut River), 77 cm. long, the 

 caudal fin extends forward along tlie dorsal side 7*5 cm., sloping 

 downward nearly to the body, then tlie second dorsal rises abruptly and 

 runs orad 16 cm., where it reaches by a gentle slope the general out- 

 line of the body. There follows a finless space ;1"5 cm. in length 

 which is succeeded by the first dorsal, whose extent is 9 cm., being 

 therefore shorter as it is lower than the second dorsal. The anus is 

 opposite the orad part of the second dorsal. 



The fins, therefore, take up almost the whole of the hinder half of 

 the mid-dorsal line. In Myxine only one-sixth was thus occupied. 

 On the ventral side we have only the caudal, extending about as far 

 here as it does above. 



These fins are sup})orted by a series of chondroid rays, lying quite 

 close to one another in the median plane. They are straight and slope 

 aborad from the fatty-fibrous ridge-pole of the myelonal canal (PI. 

 XLIX, fig. 1, a,) to the very edge of the fin. They are found in all the 

 fins. Their form is represented in PI. XLIX, fig. 1, where only one 

 ray is drawn complete. As shown, it bifurcates twice and thus ends 

 distally in four fine branches. This figure is IVom the central p.'irt of 



