xii PHYLUM CHORDATA 367 



covered closely with very minute hard placoid scales or 

 dermal teeth somewhat larger on the upper surface than 

 on the lower. These are pointed, with the points directed 

 somewhat backwards, so that the surface appears rougher 

 when the hand is passed over it forwards than when it is 

 passed in the opposite direction. When examined closely, 

 each scale is found to be a minute spine situated on a 

 broader base. The spine consists of dentine covered with a 

 layer of enamel ; the base is composed of bone, and the 

 whole scale has thus the same essential structure as a tooth. 

 Along each side of the head and body runs a faint depressed 

 longitudinal line or slight narrow groove, — the lateral line. 



As in fishes in general, two sets of fins are to be 

 recognised, — the unpaired or median fins, and the paired or 

 lateral. These are all flap-like outgrowths, running vertically 

 and longitudinally in the case of the median fins, nearly 

 horizontally in the case of the lateral ; they are flexible, 

 but stiffish, particularly towards the base, owing to the 

 presence of a supporting framework of cartilage. Of the 

 median fins, two — the dorsal — are situated, as the name 

 indicates, on the dorsal surface : they are of triangular 

 shape ; the anterior, which is the larger, is situated at about 

 the middle of the length of the body, the other a little 

 further back. The caudal fringes the tail ; it consists of a 

 narrower dorsal portion and a broader ventral, continuous 

 with one another round the extremity of the tail, the latter 

 divided by a notch into a larger, anterior, and a smaller, 

 posterior lobe. The tail is heterocercal, i.e., the posterior 

 extremity of the spinal column is bent upwards and lies in 

 the dorsal portion of the caudal fin. The ventral or 

 so-called anal fin is situated on the ventral surface, opposite 

 the interval between the anterior and posterior dorsals ; it 

 resembles the latter in size and shape. 



