xii PHYLUM CHORDATA 365 



teeth. An air-bladder is frequently present, and in certain 

 exceptional cases acquires the function of a lung or chamber 

 for breathing air. 



Sub-class I. Elasmobranchii 



A Dog-fish may be selected as a convenient example of 

 the sub-class and of the class Pisces. Dog-fishes occur at 

 slight depths off the coasts in all quarters of the globe. 

 The commonest British forms are the Rough Hound 

 (Scy Ilium canicula), the Lesser Spotted Dog-fish (S. catulus], 

 the Piked Dog-fish (Acanthias vulgaris], and the Smooth 

 Hound (Mustelus vulgaris). Allied species of the same 

 genera occur in the Southern Hemisphere. For the de- 

 scription which follows any of these species will be found to 

 serve very well. 



A brief general account of the Dog-fish has already been 

 given in the introduction to the Craniata : this has now to 

 be extended and supplemented. The general shape (see 

 Fig. 192, p. 328) may be described as fusiform; at the 

 anterior or head end it is broad and depressed ; posteriorly 

 it tapers gradually and is compressed from side to side. 

 The head terminates anteriorly in a short blunt snout. The 

 tail is narrow and bent upwards towards the extremity. The 

 entire surface is 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 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 



