214 A VERS A ND J A CKSON. [Vol. XV 1 1 . 



same alternate variation was true of the twelve to thirteen 

 variety. 



In 162 specimens collected by Mr. Jackson in Monterey 

 Bay in July, 1897, the following variations were found: 



162 



The variation in the number of gills and hence of gill carti- 

 lages is, therefore, in no way dependent upon the formation of 

 the oesophago-cutaneous duct, which is always upon the left 

 side. 



Neither does the variation bear any constant relation to the 

 size or sex of the specimen. The average length of the 162 

 specimens was 18.3 inches (varying from 14 to 23 inches). 



The lo-gilled form measured 19 inches, while one of the 

 13-gilled forms measured only 17 inches. 



Behind the gill region there is a considerable portion of the 

 body devoid of any skeleton, excepting those structures already 

 mentioned in connection with the notochord. In the posterior 

 region of the body, however, we find the large caudal fin sup- 

 ported by an extensive set of cartilages (PI. XXIII, Fig. 14), 

 The caudal fin arises about the middle of the body in the median 

 dorsal line as a slight ridge of the integument, which gradually 

 increases in height and is well developed posteriorly. It passes 

 around the posterior end of the body and terminates on the 

 ventral side in the median line just behind the cloaca. 



For convenience of description, the caudal fin may be divided 

 into a dorsal fin and a ventral fin (PI. XXIII, Fig. 14, DF, 

 VF). Each of these is supported by a number of slender, car- 

 tilaginous fin-rays, the most of which are fused at their proxi- 

 mal ends with a pair of longitudinal bars, dorsal and ventral. 



