of the Carboniferous Period. 3 



abounds in some parts of the black impure beds intercalated 

 between the carboniferous limestone and overlyiug shale of Der- 

 byshire, from whence the specimens described were collected by 

 W. Hopkins, Esq., who presented them, with a suite of fossils 

 from that district, to the University collection at Cambridge. I 

 have great pleasure in dedicating it to one who, as a mathema- 

 tician, and as an observer in the field, has so materially advanced 

 the science of geology. 



Isodus leptognathus (M'Coy). 



I provisionally apply the above name to a dentary bone of a 

 Ganoid fish apparently allied to Glyptolepis. It is destitute of 

 ornament, about 2 inches long and 3J 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.) 



Centrodus (M^Coy), n. g. 

 (Etym. Kevrpov, galli calcar, and ohov^y 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 



1* 



