236 WALKS AND TALKS. 



XL/I. THE DYNASTY OF" KISHKS. 



DEVONIAN AND CARBONIFEROUS TIMES- 



WHEN the morning of the Devonian Age dawned, a new 

 form was seen moving in the populous sea. It was a verte- 

 brate form. Without a bony skeleton, its cartilaginous frame- 

 work and general plan embodied a new conception. Among 

 vertebrates its organization was decidedly low; but it was not 

 a fish in any ordinary acceptation of the term, though w"e 

 shall have to call it a fish. There were other vertebrate forms 

 more clearly fish-like, but all widely separated from modern 

 fishes. One could easily distinguish three types of these 

 archaic vertebrates. They are known among us as E-las'-mo- 

 brancJis, Plac'-o-derms and Gan'-oids. The Elasmobranchs are 

 a group which still survives. They are all shark-like. The 

 kinds which lived in the Devonian were true sharks (Sel'-a- 

 choids) of the peculiar Ces-trac'-i-on family, the best known 

 species of which ranges from Japan to New Zealand. Cestrac- 

 ion, the Port Jackson Shark, has spines in front of both the 

 dorsal fins; the nostrils unite in the cavity of the mouth, and 

 the upper lip is divided into seven lobes. The teeth along 

 the middle of the mouth are small. External to these are 

 large flat teeth twice as broad as long, arranged in oblique 

 series so as to form a sort of tesselated crushing surface. 



Among the very earliest American fishes were some of 

 these spine-bearing sharks. The spines are flattened, two- 

 edged like a bayonet, and curved as if one had belonged to 

 the right side and the other to the left. The external surface 

 was covered with a thin coating of enamel, sometimes smooth, 

 sometimes ornamented. These spines are not attached by a 

 joint, but inserted in a mass of cartilage imbedded in the 

 flesh. They were perhaps the front rays of the pectoral fins. 

 Some of them were more than a foot in length. Being two- 

 edged and very sharp, they must have been very powerful 

 weapons, offensive or defensive. These cestracionts were 

 numerous during the Corniferous period. Their smooth, 

 brown spines are very often found in the Corniferous lime- 



