160 TJniversiiy of California Publications in Zoology [Vol. 13 



spines worn off so that, although they are of the dorsal type, super- 

 ficially they too in the adult appear as spade-shaped scales roughly 

 resembling the ventral type. 



It may then be concluded that in general the dorsal and ventral 

 scales represent distinct types with morphological differences depend- 

 ent largely upon the location of the scale. In other words, an indif- 

 ferent scale (fig. E) located in a position in which it is subjected to 

 but slight pressure may by the development of a posterior arm force 

 the spine sharph^ upward and thus assume the dorsal form. On the 

 other hand, a scale located ventrally in the same segment and subjected 

 to pressure has the posterior arm largely or entirely suppressed ; in 

 such a scale the lateral arms greatly spread out, whereupon the scale 

 takes on the spade-shaped form. 



VIII. Variations in Size of Scales 



1. Variations in Size Due to Time of Origin. — It will be observed 

 that in several of the patterns (pis. 8 and 9) tw^o distinct kinds of 

 scales are seen. These may be distinguished as: (1) One or more 

 large embryonic scales or spinules, either of the dorsal or of the ventral 

 type; and (2) smaller secondary scales which make up the majority 

 of the scales in the pattern. 



(a) The embryonic spinules are present on the body of a young 

 heterodont which has but recently left the shell as enlarged protuber- 

 ances extending along the back from the postaural division of the 

 lateral line system backward between the lateral lines to slightly 

 beyond the second dorsal fin. Back of the cloacal region a few of 

 them are found almost completely to encircle the body. Embryonic 

 spinules are also to be found between the pelvic and the pectoral fins, 

 along the supraorbital crests and on the-ventral part of the body from 

 the symphsis of the lower jaw to the tail. 



(&) The secondary scales appear among, and later than, the em- 

 bryonic spinules ; they occur in addition on the sides of the body and on 

 the fins where no embryonic spinules form. Among the secondary scales 

 great variation in time of appearance, and hence in size of the scale 

 is evident. Not infrequently a single area, as we have seen, reveals 

 a complete series ranging from small immature scales which have just 

 broken through the integument to large ones which have attained to 

 the Greek-cross type. As all of these come to maturity in the adult, 

 a uniformity in size obtains. 



