1914] Daniel: The Anatomy of Heterodontus francisci 153 



IV. The Form of the Dorsal Scale 

 A dorsal scale taken from the protected region under the base of 

 the anterior dorsal fin presents the appearance of a cross fourchee, or 

 more simply a Greek cross upon which an unusually high and sharp 

 pointed spine arises {sp., fig. C, see also pi. 9, fig. 5). The four arms 

 of the cross are similar, and are placed essentially at right angles to 

 one another. Distally the arms tend to bifurcate, thus spreading into 

 broader and more efficient supports. Surmounting the arms and the 

 spine are pronounced crests {cr., cr.' , and cr.'^) forming the dividing 

 lines between surfaces equally as pronounced. The primary or axial 

 one of these crests {cr., fig. C) appears as an inverted Y, the limbs of 

 which bound the wedge-shaped groove {gr.) on the anterior arm. The 

 two limbs of the Y unite and extend backward and upward as its 

 stem to the tip of the spine, giving off symmetrical pairs of secondary 

 crests. The most constant of the secondary crests is the pair which 

 runs outward right and left to the lateral arms of the cross {cr/), 

 breaking up distally into two or more divisions. These may be desig- 

 nated as the lateral crests. One or even two secondary crests {cr.") 

 may arise on and partly encircle the anterior arm of the cross. 



post, a 



f^ot-.u. 



^ 



j \ JP^ 





^ 





col^S\^ 





)i'3=^^>pf^ „ ^-"---^ 





ii\v^^ *"" '""'"^ 





\f 



S^jMfSs/^^ ant.a -V2 





■ ' V 



yi/y\^^^^ -. * ^==^ 



lat. a/ 



\J 



dr. -^--^--'"" 



post. a. 



-ped. 



lat. 



C D 



Figs. C-D. — Dorsal type of placoid scales of Heterodontus francisci. X 27. 

 C, dorsal view; D, lateral view. ant. a., anterior arm; ba., base of scale; col., 

 column on base; cr., axial crest; cr.', lateral crest; cr.", secondary crest; gr., wedge- 

 shaped groove on anterior arm; lat. a., lateral arm; ped., pedicel or neck; post, a., 

 posterior arm; sp., spine. 



All that we have seen represents only the body of the scale. Buried 

 deeply in the integument of the adult is the much larger base {ha.) 

 which, when dissected out, as in figure C, may be seen to be rhombic 

 in form, and more compact than the base of a scale of the ventral type. 



