1914] Daniel: The Anatomy of Heterodontus francisci 157 



evident crests which enclose the wedge-shaped groove. The median 

 crest has practically reached the tip of the spine, and the termini of 

 the secondary crests are seen on the lateral arms. 



In the fourth scale (fig. H) the principal change is noted in the 

 development of prominent lateral crests. While the posterior arm has 

 advanced considerably, the general shape of the scale is little changed 

 from that of the figure just preceding. 



The fifth scale (fig. I) is an accentuation of the one just described. 

 The principal change here seen is in the broadening out of the pos- 

 terior arm, by which the angles between it and the lateral arms more 

 nearly approach right angles. In this stage a secondary pair of lat- 

 eral crests appears, the right and left parts of which pass around the 



post. a. 



lata. 



~~-~^an.t.a. 



Figs. H-J. — Dorsal scales showing increase in complexity of the dorsal type 

 of scale through the stages H and I to the adult scale J. For explanation of 

 parts see figures C-D. 



sides of the anterior arm from the median crest. The whole scale 

 clearly has but a lower degree of specialization than has the last scale 

 of the series, which in turn is in most respects like a typical dorsal 

 scale in a protected region. 



In a comparison of figures I and J a change in the position of the 

 lateral crests {cr'.) is noticeable. In J the lateral crests, by a back- 

 ward growth of the lateral arms together with a lateral development 

 of the posterior arm so as to give background to the spine of the scale, 

 have come to lie more dorsal in position. 



That the scales herein described do not constitute a new type, but 

 are immature scales of the dorsal type, is suggested by the fact that 

 we have been able to trace a complete series of transitional stages from 

 the triangular to the complex dorsal type within a single area. But 

 were our information confined to the immature specimen described, 



