150 University of California Publications in Zoology [Vol. 13 



II. Types op Scales 



In general two types of scales prevail. These we may designate 

 from the position which they take on the body as ventrals and dorsals, 

 although modifications of the latter may pass laterally and even ven- 

 trally. The ventral scales occupy an area under the body which in 

 the adult is considerably wider than in the young. They are distrib- 

 uted in an anteroposterior direction from the symphysis of the lower 

 jaw to the caudal region. In the area immediately surrounding the 

 cloaca, however, no scales are present. 



The dorsal scales are prominent structures in a young heterodont 

 which has just assumed its protective exoskeleton. They are here 

 distributed dorsally above the lateral line as far back as the segment 

 to which the pelvic fin is attached. At this place and posteriorly 

 scales which are modified dorsals 'pass far down the sides so that be- 

 hind the pelvic fins a few may be foimd which encircle the body, 

 except over a very narrow area in the mid-ventral line. Types of 

 scales and numerous modifications of these we shall now consider. 



III. The Form op the Ventral Scale 



In our study we may consider as a type for examination one of 

 the ventral scales located in the mid-line under the chin (fig. A, and 

 pi. 8, fig. 2). Such a scale when seen from above is characteristically 

 spade-shaped. It possesses a sharp spine {sp., fig. A) which, in its 

 natural position, projects backward and downward, while it is pro- 

 vided with an anterior arm (ant. a.) which is more or less imbedded 

 in the integument. Two lateral wings or arms {lat. a.) are also present, 

 which anteriorly are separated from the anterior arm by a more or 

 less abrupt niche, and which posteriorly extend in a broader curve 

 to the tip of the spine. 



When examined in detail, certain finer parts are seen to be present. 

 At the tip of the anterior arm there are two crests (cr.), which curve 

 upward and backward over the margins of the arm. Between these 

 crests there is a wedge-shaped groove {gr., fig. A) characteristic of 

 the scale of Heterodontus. On the margins of the lateral wings of the 

 spade are indications of other crests which may fail to reach the spine. 

 Centrally, in a mature scale, tubules or dentinal canals (dent, c.) maj 

 be seen to radiate from a median pulp cavity (p. cav.). 



