1914] 



Daniel: The Anatomy of Heterodontus francisci 



151 



In addition to the parts of the scale appearing above the surface, 

 there is a larger, deeply imbedded support, the base {ha., figs. A and 

 B) which in the adult is buried beneath the integument. The base is 

 roughly diamond-shaped, with the posterior and lateral limbs pro- 

 jecting much farther than does the anterior limb. The whole scale 

 may be seen to advantage in side view. 



dent c. 



ant.a.- 



A B 



Figs. A-B. — Ventral type of placoid scales of Heterodontus francisci. X 27. 

 A, dorsal view; B, lateral view. ant. a., anterior arm; &a., base of scale; col., 

 column on base ; cr., axial crest ; cr.', lateral crest ; dent, c, dentinal canal ; gr., 

 wedge-shaped groove on anterior arm; lat. a., lateral arm; p. cav., pulp cavity; 

 ped., pedicel or neck; sp., spine. 



In side view the ventral scale of Heterodontus, like that described 

 by Hertwig for AcantJiias, appears not unlike a helmet (see fig. B). 

 In such a view the body is seen to be flattened and the spine, like the 

 plume of a helmet, points sharply backward and slightly upward (in 

 normal position downward). The anterior arm is seen to rise consid- 

 erably, leaving between its crests and the overhanging lateral wings 

 a slight concavity. Connecting the body of the scale with the base 

 appears the neck or pedicle {ped., fig. B). This is modified anteriorly, 

 posteriorly, and laterally by columns which continue down the neck 

 and outward towards the angles of the base. 



The base itself, in such a view, shows its concave nature. Passing 

 to the concavity in fresh material are seen fibres of connective tissue 

 which penetrate the hard substance and bind the whole scale to the 

 deeper layers of the integument. 



Modifications in Form of the Ventral Type of Scale. — Various 

 modifications of the ventral type of scale are present. Among these 

 are some which I shall designate as stomodeal denticles (pi. 8, fig. 1). 



