32 



THE ELASMOBRANCH FISHES 



general like that of Heptanchus. In some, however, a pattern exists only over 

 limited areas. In others the scales may be confined to rows along the back, or, 

 in addition to this arrangement, they may be scattered more or less pro- 

 miscuously over the body, as they are in some of the rays. In other rays integu- 

 mentary scales may be entirely lacking (Myliohatis, Trygon). 



The individual placoid scale may bear but a single spine as in Must el us 

 calif ornicus (fig. 34a), CarcJiarias, Pristiophorus. Or it may be tricuspate, 

 multitudes of scales covering the surface as in Heptafichus (fig. 27a), Scyl- 

 lium, Zygaena, Pristiurus, and Pentanchus. The scales may be found in geo- 

 metrical exactness and beauty as in Sqnaliolus (fig. 34b) , or they may present 

 various designs from a simple spade-shape to the anchor scale (fig. 34c) or to 

 the complex Greek cross of Heterodontns. The gross anatomy of the type 

 described for HeptancJiiis may be taken as an example of more or less gen- 

 eralization. 



FINEE ANATOMY OF SCALE 



A section through a placoid scale of Scymnus (fig. 35) illustrates the finer 

 structure characteristic of the Elasmobranch scale. In such a section the crest 

 surmounting the main body continues backward to the spine. The base is rela- 



atively large and a pedicel or 

 neck connecting it with the 

 body of the scale is practically 

 lacking. The base is fixed to the 

 corium by connective tissue 

 and is perforated by a central 

 canal (c.c.) which leads into a 

 large median pulp cavity 

 ip.c.) An anterior and a pos- 

 terior canal lead off from the 

 pulp cavity; and from the an- 

 terior and posterior canals, as 

 well as from the pulp cavity 

 itself, smaller dentinal canals 

 (d.c.) extend into the dentine. The enamel (e.) surrounding the exposed part 

 and sinking slightly into the integument is much better developed anteriorly 

 than it is posteriorly. 



Considerable doubt has been raised as to whether or not the so-called enamel 

 of the placoid scale and of the tooth of the Elasmobranchs is in fact compara- 

 ble to the enamel of the teeth of higher forms. The studies thus far made show 

 that the enamel formed in the Elasmobranch fishes presents a variety of types. 

 In the rays it appears to be in all essential respects true enamel ; in some of the 

 sharks the evidence is not so clear. That the dentinal tubules in a type like 

 Galeus may be traced far into the outer layer shows that there is here no clear 

 demarcation between dentine and enamel. For the present, then, we may 

 think of the harder outer layer of the scale in some of the Elasmobranchs as 

 composed of a substance just beginning to differentiate into true enamel. 



Fig. 35. Sagittal section showing finer anatomy of 

 placoid scale, Scymnus lichia. (From O. Hertwig.) 



c.c, central canal; f/.c, dentinal canals; e., enamel; 

 p.c, pulp cavity. 



