88 A HISTORY OF FISHES 



among a shoal of fish, and, wielded with a side-to-side move- 

 ment, inflicts great execution. When attacking larger prey, 

 the Saw-fish is said to use the saw to tear off lumps of flesh 

 from the body of the victim, the detached fragments being 

 seized by the mouth and swallowed at leisure. 



It is of interest to note that in the Chimaeras and their 

 allies {Holocephali) , although the skin is naked in the adult, 

 small patches of denticles, essentially similar in structure to 

 those of the Sharks and Rays, still remain on the claspers 

 (Fig. 80), and in the young there may be a double row of 

 denticles along the back. 



The scales of all the Bony Fishes differ from the denticles of 

 the Selachians, not only in their structure, but also in being 

 derived entirely from the dermal layer of the skin (Fig. 37B). 

 Since the epidermis plays no part in their development, enamel 

 no longer enters into their make-up. The ancestors of prac- 

 tically all living Bony Fishes, the Palaeoniscids, ranging from 

 the Lower Devonian period to the end of the Jurassic, had the 

 body completely invested in an armour of shining bony plates, 

 arranged in regular parallel, oblique and longitudinal series 

 (Fig. 127). These "ganoid" scales represent the most primitive 

 type known in Bony Fishes, and persist to-day in the Bichirs 

 (Polypterus) and in the Sturgeons {Acipenser) and their allies. 

 Those of the Bichir take the form of juxtaposed plates, roughly 

 rhomboid in shape, articulated with one another by a kind of 

 peg-and-socket joint between the upper and lower edges of 

 adjacent plates. Each scale is made up of three distinct layers; 

 on the surface is a shining, enamel-like substance called ganoine 

 (from which the term ganoid is derived) ; within is a thick layer 

 of bone; and between the two another substance known as 

 cosmine, containing minute blood-vqssels. Where flexibility 

 of the body is a consideration the advantage of jointed plates 

 over solid armour is obvious, and the actual shape of the first 

 scales was probably determined mainly by purely mechanical 

 factors. The scales are in close connection with the underlying 

 muscles, and when the flexures of the trunk and tail in 

 swimming cause these muscles to contract the skin tends to be 

 wrinkled into definite circumscribed areas. 



In the existing Sturgeons [Acipenser), clearly descended from 

 the Palaeoniscids mentioned above, the sole remains of the 

 elaborate armour plating is a patch of small ganoid scales on 

 the upturned part of the tail (Fig. 26a). There are, however, 

 five widely separated rows of bony scutes or bucklers running 



