NORTH STAFFORDSHIRE COAL FOSSILS. 215 



ORDER PLAGIOSTOMI (PLACOIDEI, AGASSIZ). 

 FAMILY CESTRACIONTID^B, AGASSIZ. 



GENUS CTENACANTHUS, AGASSIZ. 



Ctenacanthus hybodoides, Egerton. (Ref., Quar. Jour. Geol. 

 Soc., vol. ix., pi. 12). The defensive spine known by this 

 name is about ten inches in length, including about two 

 inches of the base, which was inserted in the integuments. 

 The greatest width is about one-and-a-half inch. " On 

 quitting the body of the fish this spine is nearly straight 

 for about half its length : it is thence slightly re-curved. 

 The angle it forms with the line defining the integumentary 

 investment is about 45." The exposed portion of the spine 

 is ornamented with parallel, longitudinal, rounded ribs. 

 In several of our specimens there are twenty of these ribs 

 near the base, diminishing to about ten at the apex. 



Associated with Ctenacanthus we have found teeth which 

 are closely allied to the genus Cladodus (see plate, fig. 15). 

 The base of the tooth is narrow, and slightly concave from 

 above, downwards : from the superior surface there spring 

 five or more denticles. The principal denticle is near the 

 centre of the tooth and is the largest. The secondary 

 denticles are arranged on each side of the central cone. 

 Sometimes there are two on each side ; in others, there 

 are three on one side and one on the other. The secondary 

 denticles all decrease in size as they recede outwards from 

 the principal cone. The denticles are pointed, and the 

 the surface striated with coarse thread-like striae running 

 from base to apex. Numerous dermal processes have been 

 found associated with these remains. These, as also teeth, 

 are similar to those discovered with Ctenacanthus by Mr. 

 Thomson in the Airdrie coal-measures. (Trans. Geol. Soc. 

 Glasgow, vol. iv,) 



