THE STATE GEOLOGIST. 



49 



pointed cusps adapted for piercing, and the anterior dorsal fin 

 appears to have been armed with a powerful spine similar to those 

 described under the name of Ctenacanthus. This Devonian genus, 

 as has been said, is the most primitive type of Elasmobranch yet 

 discovered, and is regarded as the ancestral form from which a 

 host of Carboniferous and most modern sharks are derived. A 

 curious form intermediate between sharks and rays (Tamiobatis) 

 is also known from the Devonian ; and if we may assume dental 

 plates to furnish a reliable clue, chimaeroids (Ptyctodus) were 

 present throughout this system in astonishing abundance. 



During the Carboniferous the group of Elasmobranchs in- 

 creased prodigiously in point of numbers, size and variety, and 

 attained a world-wide distribution, but their rapid culmination 

 which took place at the opening of this era was followed toward 

 its close by an equally notable decline, approaching almost to the 

 verge of extinction during the Permian. Some of the Carbonifer- 

 ous sharks were formidably armed, the largest fin-spines and most 

 powerful crushing, cutting and piercing- teeth known to the science 

 of ichthyology having been developed during this era. An inter- 

 esting generalized shark from the French Coal Measures {Pleura- 

 canthus) combines within itself such a variety of synthetic char- 

 acters as to justify the observation that "it is a form of fish which 

 might with little modification become either a selachian, dipnoan, 

 or crossopterygian." The long-lived group to which the Port 

 Jackson shark {Cestracion) Fig. 6, belongs was exceedingly 

 plentiful during the Carboniferous, and the number of species very 



Port Jackson shark, Cestracion philippi (female). Recent; Australia 

 (From Dean, after Garman.) 



4 GEOL 



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