EXPLANATION OF PLATES 



All of the figures in plates 8 and 9 are from an immature specimen of 

 Heterodontus francisci. This stage was selected rather than that of an adult or 

 of a still younger fish because of its significance in explaining the differences 

 existing between the young and the adult form. 



PLATE 8 



All figures magnified 18.8 diameters 



Fig. 1. — Eepresents a pattern of scales from the floor of the buccal cavity 

 near the symphysis of the lower jaw. The scales or stomodeal denticles, which 

 are modifications of the ventral type of scale, are here grouped around one of 

 the numerous sense buds found in the mouth. 



Fig. 2. — A pattern showing the ventral type of scale. In the middle of this 

 pattern there is present one of the enlarged embryonic spinules. 



Fig. 3. — A pattern taken from the ventral side of the body posterior and 

 lateral to the cloaca. In this occur three of the enlarged anchor scales, one of 

 which is an embryonic spinule. While the anchor scales are of the modified 

 dorsal type, the remainder of these scales develop into the adult ventral type 

 of scale. 



Fig. 4.— Represents an area of integument extending from the ventral side 

 of the pectoral fin over the margin to the dorsal side. Scales at the top of the 

 figure are of the modified ventral type and those at the bottom are of the 

 dorsal type. Located between these two are the marginal scales which through 

 exposure become greatly enlarged. 



[164] 



