FISH OF THE CARBONIFEROUS PERIOD. 6 



ornament, about 2 inches long and 3i lines deep at the broad 

 end ; it is slightly curved and tapers a little to a rounded extre- 

 mity at the anterior end ; the upper margin is minutely rough- 

 ened and contains about thirty nearly equal teeth, nearly their 

 own length apart, conical, about one line long and half a line 

 wide at base ; the upper portion is smooth and the base coarsely 

 fluted as in Rhizodus, of which it also possesses the important 

 internal character of the medullary cavity being conical and 

 simple in the upper or smooth part of the tooth, and as it ap- 

 proaches the base abruptly branching into several root-like pro- 

 cesses, one corresponding to each of the external flutings : from 

 Rhizodus however the genus is distinct by the teeth being of 

 nearly uniform size (not of two distinct sizes), and by the section 

 of each tooth being circular instead of elliptical with cutting 

 edges. 



From the yellow sandstone shale of Moyheeland, Draperstown 

 (Ireland). 



(Col. Mr. Griffith at Dublin.) 



Centrodiis (M^^Coy), n. g. 

 (Etym. KevTpov, galli calcavj and 6Sov<^j dens.) 

 Gen. Char. Tooth simply conical, gradually tapering, slightly 

 curved backwards, apex pointed, section circular throughout ; 

 medullary cavity large, conical, simple, so wide at base that the 

 tooth is reduced to a thin edge; surface even, very finely striated 

 longitudinally. Microscopic structure : — exceedingly fine calci- 

 gerous tubes radiating directly from the pulp-cavity towards the 

 periphery, near which they terminate in numerous very minute 

 calcigerous cells, beyond which is a narrow clear layer bounded 

 by a definite dark line, outside of which is a coating of glass- 

 like enamel, without perceptible organic structure under a 

 power of 300 diameters. 



Externally these teeth slightly resemble Rhizodus (Ow.), but 

 are distinguished by their circular section, and wide, simple pulp- 

 cavity, which latter distinguishes it from nearly all palaeozoic teeth 

 except the old red sandstone genus Cricodus (Ag.), from which 

 it is known by its more slender conical form, and wanting the 

 strong longitudinal ridges of the surface, as well as possessing 

 the distinctly defined enamel layer, which has all the appearance 

 of having been secreted by a distinct organ, and is quite difi'erent 

 from the condensed dentine which forms the false enamel of 

 most fish-teeth. 



Centrodus striatulus (M'Coy). 



Sp. Char. Tooth about half an inch long and IJ line in diameter 

 at base ; conical, gradually tapering to the pointed apex, with 



a2 



