PEDIGREES. 181 



accompanied by degeneration. The oldest of the 

 ancestral forms belongs to the genus Cheirolepis, 

 and occurs in the Old Red Sandstone (fig. 15 A). 

 One of the most striking features of this fish was 

 its covering of scales. These were exceedingly 

 small, and closely fitting, but they did not over- 

 lap. The head was enveloped in bony plates, 

 and the mouth was large, at the front of the head, 

 instead of on the under surface after the fashion 

 of modern sharks, and armed with teeth. These 

 characters stand in strong contrast with the 

 typical modern sturgeon, wherein the body is 

 covered, not with closely-fitting scales, but with 

 rows of isolated bony bosses arranged, one along 

 the back, one along each side, and one along 

 each side of the under surface. Again, in the 

 living sturgeon the mouth has shifted to the 

 under surface of the head, and the jaws have 

 lost the teeth, the mouth now being suctorial. 

 But it is interesting to note that in the embryo 

 sturgeon the jaws bear teeth. 



But there are other sturgeons which serve as 

 links in the chain which we hope will one day 

 be complete enough to carry us back by easy 

 transitions from the toothless and curiously 

 armoured form, which we have just discussed, 

 to the toothed and scaly members of the genus 

 Cheirolepis. These links are, however, it must 

 be admitted, somewhat slender. 



The most interesting are the living shovel- 

 beaked sturgeons of the genus Polyodon. In 

 many respects they are, like their more familiar 

 cousins, the sturgeons of the genus Acipenser, 

 both highly specialised and degenerate. They 



