﻿PTYCTODONTID-E. 37 



under each tooth * is conclusive proof of their bearing no relation to 

 the familiar membrane-bones thus named in higher fishes. 



Synopsis of Families. 



I. [Imperfectly defined. Spines unknown.] 



One pair of dental plates above and 



below PTYCTODONTID.E (p. 37). 



II. Dorsal fin-spines absent. Rostral spine 

 in male. 



Trunk depressed, snout elongated. Two 

 pairs of dental plates above, one pair 

 below , Squalorahd^: (p. 40). 



III. Spine in front of anterior dorsal fin. 

 Rostral spine in male. 



Few dermal plates on head. Two pairs of 



dental plates above, one pair and an 



anterior azygous tooth below Myriacanthid^; (p. 43). 



No dermal plates. Two pairs of dental 



plates above, one pair below Chim^erid^: (p. 52). 



Family PTYCTODONTID.E. 



A family at present indefinable, of doubtful ordinal position, 

 known only by remains of the dentition. A single pair of large, 

 laterally compressed, dental plates in each jaw, meeting at the 

 symphysis and with few tritoral areas. 



The genera of this family have not hitherto been defined, even so 

 far as existing materials will permit. There are as yet no examples 

 of the teeth in the collection of the British Museum ; but an exami- 

 nation of a large number of Russian specimens in St. Petersburg, 

 American specimens in New York, recently discovered examples 

 from Canada in the Geological Survey Collection at Ottawa, and 

 several undescribed forms from the Eifel Devonian in the Museum 

 of Comparative Zoology, Cambridge (Mass.), has suggested to the 

 writer the following provisional arrangement. 



Synopsis of Genera. 



I. Symphysial surface narrow; tritors more or 

 less laminated. 

 Oral surface triturating, the tritors being 

 well differentiated and consisting of 

 hard, punctate, superimposed laminas, 

 arranged obliquely to the functional 

 surface Ptyctodus, Pander. 



1 K. Owen, Odontography, p. 65. 



