Dentition 0/ Pleuroplax (Pleurodus), A. S. Woodw. 293 



of the Helodont type. A representation of the slab is given 

 on PL XIII. fig. 2, and drawings of the teeth, natural size 

 and enlarged, arc also given (figs. 2a-2(j). The surface of 

 all the teeth is enamelled and ornamented by minute punc- 

 tures, indicating the superficial extremity of the nutritive 

 canals. 



These specimens are important as affording positive evi- 

 dence of the structure of another group of Cochliodonts. 

 Until the description of the dentition of Psephodus magnus 

 by Traquair* was rendered possible by the discovery of the 

 East-Kilbride specimen, very little reliable information 

 respecting this family was accessible. The dentition of 

 Pleuroplax as exhibited in these specimens confirms the opinion 

 expressed by Traquair that the teeth of the genus Lophodus 

 of Rowanowski were merely accessories in the dentition of 

 other genera ; and the statement of Sir R. Owen f in 1867, 

 " that it would seem as if the several teeth of each oblique 

 row in Cestracion had been welded into a single dental mass 

 in Cochliodus" may well be applied to the whole of the family 

 of the Cochliodonts ; the broad palatal teeth of Pleuroplax 

 clearly indicate that their present form and construction is 

 due to the ankylosis of the smaller series of teeth possessing 

 the Lophodont character which still remain separated in the 

 anterior parts of the mouth. 



The occurrence of the teeth of Helodus on the slab from the 

 Staffordshire Coal-field in conjunction with the spine of 

 Pleuroplax, referred to in my paper J on Pleurodus affinis, 

 seems to point to the inference that Helodus simplex must 

 also be considered a Lophodont and absorbed in other 

 genera. This view is confirmed by Dr. Traquair §, who has 

 pointed out that " a fine series of specimens of Helodus simpler, 

 Ag\, in the collection of Mr. John Ward, F.G.S., Longton, 

 clearly shows that the teeth in this species have the form of 

 1 Lophodus J that the entire dentition consisted of teeth gene- 

 rally similar in shape, and that the dorsal fins were armed 

 with spines resembling those of Pleurodus.' 1 '' Mr. A. Smith 

 Woodward || restricts the genus Helodus to the type species 

 H. simplex, Ag., " a genus still awaiting elucidation." lie 

 regards it as closely related to Pleuroplax both by the den- 

 tition and the dorsal fin-spine, and has no doubt that, in what- 



* Geol. Mag. dee. iii. vol. ii. p. 340, pi. viii. (1885). 



t Geol. Mag. vol. iv. p. ">'.). 



j Op. cit. p. 182. 



§ Geol. Mag. dee. iii. vol. ii. p. 344, 1885 (footiiotej, and vol. v. p. 84. 

 1888. 



|| ' Catalogue of the Fossil Fishes iu the British Museum,' part. i. 

 p. 171 (1889). 



