PEDIGREES. 175 



There is nothing very exciting to relate about 

 Chimseroids. They are very ugly fish. Their 

 claim, however, to attention is a strong one, for 

 there seems to be no doubt that they form a 

 connecting link with the dipnoi. The backbone 

 is not divided into separate bony segments, but 

 is represented by that more primitive structure, 

 the forerunner of the typical backbone, known 

 as the notochord (p. 170). There is but a 

 single gill opening, as in the dipnoi (lung-fishes). 

 But there are two particulars which lift the 

 chimeras out of thq ranks of the commonplace. 

 The first of these concerns the teeth. These are 

 somewhat plate-like structures, bearing hard- 

 ended areas known as ''tritors." There are two 

 pairs of these in the upper jaw, and they bear a 

 resemblance, on the one hand, to the teeth of 

 the lung-fishes, and on the other to those of 

 certain extinct sharks known as Cochliodonts. 

 The resemblance to these latter is significant, 

 suggesting that they may have had a similar 

 origin — the fusion of separate smaller teeth. 

 The second of the chimeras' notable characters 

 is represented in a remarkable movable spine 

 in the head, of unknown function, the free end 

 of which is covered with recurved spines, and is 

 received into a pit in the forehead. In some of 

 the fossil members of this group this spine was 

 represented by a sword or spike-like structure. 



About the 1 fe history of Chimseroids we know 

 next to nothing, about the embryology and 

 larval development nothing at all. 



We must turn now to the discussion of that 

 higher group of fishes, the descendants of the 



