34 



NATURAL SCIENCE. 



Rhizodontidae, and these again below the modern Polypteridae. In the 

 first family, the pectoral " archipterygium " is as much elongated as 

 in Ceratodus ; in the second family, which survived after the first 

 had become extinct, the same fin still shows the branched arrange- 

 ment of cartilages (Fig. 6), but is much abbreviated ; while in the 



Fig. 8. — Series of Diagrams illustrating the views of Dr. Anton Fritsch in regard to the stages 

 in the Evolution of the Paired Fins. 



a. Hypothetical primitive fin. 



b. Atavistic pelvic fin of aged 



female Xenacanthus. 



c. Pelvic fin of young female 



Pleuracanthus. 



d. Pectoral fin of Ceratodus. 



e. Pectoral fin of Orthacan- 

 thus. 



/. Pectoral fin of Pleiii acan- 

 thus. 



g. Pectoral fin of Xenacanthus 

 (this and the following 



figs, with fringing dermal 



rays). 

 h. i. Pectoral fins of recent 



Sharks. 

 k. Pelvic fin of Sturgeon 



(Acipenser). 



recent Polypteridae, this short lobate fin is almost or quite tribasal, 

 as in a typical modern shark. Among Elasmobranchii, the Carboni- 



