134 BULLETIN OF THE 



gar-pike ; this process gives a satisfactory explanation for the feebly 

 developed condition of the scales upon the under side of the lower jaw. 

 He observed that in the act of deglutition the floor of the mouth became 

 very much distended, so much so as to lead him to compare it to the 

 pouch of a feeding pelican. The frequent distention to which this part 

 of the skin is subjected by the greedy habits of the young fish furnishes 

 a sufficient reason why continuous plates of hard bony material are not 

 formed in this part of the dermis. The small size of the scales described 

 by Hertwig ('79, p. 9) as lying at the bases of the fins is doubtless to be 

 explained in a similar manner. Instead of being atavistic conditions, as 

 maintained by him, they would appear to be due entirely to the action 

 of purely mechanical influences. 



I have already spoken of the canals which jaierce the central part of 

 the scale, and which have been called Haversian canals by Hertwig and 

 Klaatsch. Their usual course is from below directly through the scale 

 to the upper surface, where the vessels which traverse them spread out 

 in all directions through the sub-epidermal layer. There are sometimes 

 one or two vessels of considerable size which at the upper sui-face break 

 up into small branches, but quite as often the canal is filled by a large 

 number of minute vessels apparently distinct from one another. The 

 coarse of these canals is not, however, by any means constant. I have 

 found that sometimes, instead of running directly through, they extend 

 for some distance inside the scale parallel with the surface. Indeed, in 

 the scales of one fish (287 mm. long) the rule seemed to be for quite a 

 number of canals to run horizontally through the scales for considerable 

 distances. This condition seems to me to give an additional reason for 

 regarding these canals as Haversian, or perhaps better — since the 

 osteoblasts are not arranged in concentric lamellte about them — as 

 Volkmann's canals. Figures 28 and 29 (Plate IV.) are from sections of 

 such scales. In the former figure the canal extends the greater part of 

 the length of the scale and opens to the surface at five points, near 

 those marked for.^ as can be seen in other sections not figured. 



The surface contour shows some quite marked peculiarities in several 

 instances. Figure 30 (Plate IV.) represents a portion of a scale from 

 the same fish as the one last mentioned (287 mm. long). The upper 

 surface bears numerous rounded elevations or hillocks, upon which for 

 the most part the spines are borne. The variations in shape and height 

 are indicated in the figure. Frequently the canals pierce the scale just 

 beneath these elevations and open to the upper surface at one side, as 

 indicated by dotted lines. These slightly abnormal conditions seem to 



