6 U.S. NATIONAL MUSEUM BULLETIN 2 82 



N. gyrinus this correlation also appears to correspond roughly with 

 an increase in the number of preoperculomandibular pores, forming 

 parallel geographic gradients. A prominent geographic increase in 

 number of pectoral rays, but not in pelvic rays, is found in Noturus 

 leptacanthus. The full complement of paired fin rays is probably present 

 in the yolk sac stage; the normal complement was counted in speci- 

 mens that had just lost the sac. 



Caudal fin rays. — The full complement of caudal rays is appar- 

 ently attained early in the first year of life after the yolk sac is lost 

 (fig. 1 and, see p. 32, description of the young of Noturus miurus). 

 In old specimens, there may be degeneration of one or two rays at 

 the anterior end of the fin. This is suggested by occasional short rays 

 (counted) that are neither attached at their base nor continue into 

 the body. The number of branched rays in Ictalurus and Pylodictis 

 is nearly constant, and they are differentiated at an early age, but 

 in Noturus some of the simple caudal rays apparently become branched 

 with age. The variation in number of branched rays is probably 

 because of a continuation of the branching process, in the long rays, 

 throughout life. 



That the total number of caudal rays does not change after early 

 in the first year of life is evidenced by general observation, by plotting 

 the number of rays according to standard length in several species 

 (see figs. 1 and 4), and from the fact that the rays, once developed, 

 are distinct and do not grade imperceptibly into the surrounding 

 tissues. The anterior rays often lie rather loosely inside the fin mem- 

 brane. The number of rays in each "half" of the fin also remains 

 constant. 



Enumeration of caudal rays in the species of Noturus, especially 

 in large individuals, is difficult and tedious. A few tabulations were 

 from cleared and stained specimens. Others were made by the use of 

 soft x-rays; in this method, fins were taped closely to the film holder, 

 containing high contrast film, and exposed at 22KV, 150 MAS, with 

 the x-ray unit about 20 inches from the subject. 



The method most frequently used in determining the number of 

 caudal rays is as follows: the mucus is cleaned from both sides of the 

 fin by scraping with a dull knife; the fin is then rinsed and pressed 

 flat against the glass stage of the microscope in a small pool of liquid; 

 strong light is projected through the fin and a count made; the fish 

 is then turned over to check the enumeration. 



The caudal fin can be conveniently divided into the following 

 parts : 



Upper simple caudal rays: All unbranched rays in upper part of fin. 



Lower simple caudal rays: All unbranched rays in lower part of fin. 



Branched caudal rays: All branched rays in the fin. 



