THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY. 



[SEVENTH SERIES.J 

 No. 17. MAY 1899. 



L. — On the Cretaceous Fish Plethodus. By A. Smith 

 Woodwakd, F.L.S., of the British Museum (Natural 

 History). 



[Plates XIII. & XIV. j 



In his well-known work on the ' Geology and Fossils of 

 Sussex ' (1850) Frederic Dixon briefly described and figured 

 some remarkable crushing-teeth or dental plates from the 

 Sussex Chalk, to which he gave the name of Plethodus. He 

 compared them with Ptychodus, and referred them to the 

 Cestraciont sharks. Numerous specimens were subsequently 

 discovered both in the Chalk and Cambridge Greensand, a tew 

 also in the Ganlt of Folkestone ; and when I was occupied 

 with a general survey of the English Cretaceous fish-fauna 

 in 1887 I prepared several sections to demonstrate their 

 microscopical structure. It was proved that beneath the 

 thick dense layer of vertical tubules of dentine observed by 

 Dixon there was an equally thick base of true bone with 

 numerous typical bone-lacunse. The problematical fossils 

 could not therefore be retained any longer among the Elasmo- 

 branchii. They were referred to some undetermined bony 

 fish ; and one small specimen in the Willett Collection in the 

 Brighton Museum was mentioned as displaying the Plethodus- 

 plate " so placed in the midst of a skull as to suggest its 

 connexion with the pharyngeal bones " *. During the last 



* A. S.Woodward, "A Synopsis of the Vertebrate Fossils of the 

 English Chalk," Proc. Geol. Assoc, vol. x. (1888) p. 331. 



Ann. & Mag. N. Hist. Ser. 7. Vol. iii. 26 



