1914] Daniel: The Anatomy of Heterodontus francisci 159 



the transitionals may be secondarily so modified as not to be easily 

 interpreted. 



This section is of secondary interest in another respect. It will 

 be noticed that the spines of the scales to the point X (pi. 9, fig. 8) 

 do not change direction ; hence the scales which have passed ventral 

 to the nose have the spines pointing anteriorly. X may be taken, then, 

 as the place at which reversal of direction takes place and back of 

 which the scales normally point posteriorly. 



A second interesting transition, taken at the level of the second 

 dorsal fin and encircling the body, may best be described in a recently 

 hatched embryo. The dorsal scales in such a section, although but 

 few of them are mature, are typically of the Greek-cross type. In 

 traveling downward one comes to an area of large scales which, 

 although similar to the dorsals in form, have the spines bent downward 

 so as to lie in an almost horizontal plane. These are the anchor scales 

 which we have previously considered as modified dorsals (pi. 8, fig. 3). 

 In this transitional region these scales occasionally occur down to 

 within two rows of the mid-ventral line, where in the young a narrow 

 area of small scales with long sharp-pointed spines is met. In this 

 case the dorsals have encroached so far upon the ventrals that no sharp 

 distinction between dorsal and ventral areas can be made. 



If the same transition be followed in an adult fish, striking differ- 

 ences appear. While the dorsals and their modifications are distrib- 

 uted over the back and sides respectively, spade-shaped scales occupy 

 the whole of the area under the body. In short, the ventral area which 

 in the young has a considerable sprinkling of scales of the modified 

 dorsal tj^pe, in the adult has a complete covering of spade-shaped 

 scales. 



A closer study explains this difference in the ventral scales of the 

 two stages. It will be noted that only the large scales in the ventral 

 pattern are of the modified dorsal or anchor type (pi. 8, fig. 3), and 

 that the greater number are what we may term indifferent scales, that 

 is, scales which have not yet taken on either pronounced dorsal or 

 ventral characteristics. As the ventral part of the body of the growing 

 young comes in contact with the bottom, these immature scales de- 

 velop into the flattened ventral type. As to the immature scales, we 

 may then say that there has been no actual changing from one type 

 to the other, but that there has been a progressive development into 

 the ventral type. With the anchor scales the ease is different; these 

 upon being subjected to the wear and tear of the bottom have their 



