THE ELASMOBRANCH FISHES 



25 



Modified Scales 



Scales located in a region that is more exposed may differ greatly from the 

 typical ones which we have just described. The several areas in which the 

 scales may become modified in Heptanchus are along the dorsal margin of the 

 back and especially in front of the dorsal fin, 

 around and in front of the eye, over the tip of the 

 nose and the tip of the lower jaw; in fact, over 

 any of the surfaces which are particularly ex- 

 posed as the fish swims forward. An extreme ex- 

 ample of modification is shown in figure 27b, the 

 pattern of which was taken from the anterior mar- 

 gin of the pectoral fin. The crests and even the 

 spines of the scales previously studied have here 

 disappeared. In fact the entire form has become 

 so greatly modified as to be hardly recognizable. 

 Instead of having the regular spade-shape they 

 have become enlarged nodules, the anterior and 

 posterior parts of which can be distinguished only 

 with difficulty. Not infrequently such a scale in 

 this exposed area reaches a size many times that 

 of a protected scale. 



A third type may be described briefly. Covering 

 the lining of the buccal cavity and the pharyngeal 

 box, excepting the posterior part of the roof, there 

 are numerous modified scales which are designated 

 as stomodeal denticles (fig. 27c). These denticles 

 resemble much more closely the normal scales previously described than they 

 do those secondarily modified. Indeed, the stomodeal denticles, occupying a 

 position which in the young is more protected than the normal type of scale, 

 are sharply differentiated into parts. This is especially noticeable in their 

 prominent lateral spines; in some even a second pair of laterals is present, 

 making five spines in all. It will be observed that the stomodeal denticles 

 closely resemble the lower teeth (see p. 122, fig. 121) . 



Fig. 28. Detail of plaeoid 

 scale, Hepfanclms cinereus. 

 (From Steinhard.) 



a.a., anterior arm ; ha., base 

 of scale; cr.^, primary crest; 

 cr.^, lateral crest; cr.', tertiary 

 crest ; cr.*, fourth crest ; La., 

 lateral arm; sp.\ posterior 

 spine; sp."^, lateral spine. 



