224 NORTH STAFFORDSHIRE COAL FOSSILS. 



FAMILY TRYGONID^E. 

 GENUS PLEURACANTHUS, AGASSIZ. 



Pleuracanthus (Diplodus) gibbosus. (Ref., Poiss. Foss., 

 vol. iii., tab. 22, fig. 1 5.) The generic title of Diplodus 

 was given by Agassiz to some curious teeth which were 

 discovered in 1834 in the coal shales of Silverdale. Two 

 species were named by Agassiz, D. gibbosus and D. minutus, 

 the former from Silverdale the latter from Burdie House. 

 Since the publication of Agassiz's description, specimens 

 have been found which prove that the teeth of Diplodus 

 belong to the same fish as the fin-spine named by Agassiz 

 Pleuracanthus. (See Ann. Nat. Hist., vol. xx., series 2.) 

 Professor Kner has recently given a detailed description of 

 a remarkable fish called Zenacanthus (Orthacanthus) Dechenii, 

 a fish possessing teeth and a spine which do not differ 

 generically from Diplodus and Pleuracanthus. (See Geol. 

 Mag., vol. i., p. 376.) The teeth of P. gibbosus have a round 

 base from which spring two lateral and one small central 

 denticle of unequal length. The longest of these denticles 

 is more divergent and more bent than than the rest. It is 

 sometimes on the right side and sometimes on the left, 

 according to the place it occupied in the mouth. The next 

 in size is straighter, and slightly narrower at the base. 

 The lateral edges of both denticles are often finely serrated. 

 The smallest denticle is placed between the larger ones at 

 the basal junction, and is scarcely half the length of the 

 lateral denticles. In front of this there is a large tubercle. 

 (See plate, fig. 9). We have specimens which differ from 

 the above. One of these has a narrow base from which 

 spring two straight narrow lateral denticles divergent at the 

 same angle. Between these, there are two small, slender, 

 denticles. There are several other teeth, all of which differ 



