FISH OF THE CARBONIFEROUS PERIOD. 27 



the sides ; ends abruptly truncated ; base surrounded by a 

 sharply defined thickened border ; surface smooth, highly po- 

 lished ; root nearly as deep as the crown is high, truncate be- 

 low and at the ends ; height of crown 1 line, width of base 

 S~ lines. 



This Orodus is extremely like a Ctenoptychius in general aspect, 

 from the thin, compressed, rectilinear, nearly square form of 

 crown and root, the prominent, defined ridge at the base of the 

 former and its pectinated upper edge ; this edge however instead 

 of being sharp is surmounted by a little ridge, from which the 

 small lateral ridges are given oif as above-mentioned — characters 

 which do not exist in Ctenoptychius. 



Rare : from the same locality and in the same collection as the 

 last. 



Petrodus (M'Coy), n. g. 



Gen. Char. Teeth conical, supported on a nearly circular osseous 

 base, concave beneath ; crown with a dense compact surface, 

 height not exceeding the width, deeply furrowed with rough 

 radiating ridges. Microscopic structure : vertical and horizontal 

 sections nearly similar, showing the centre to be composed of 

 exceedingly coarse irregular medullary fissures, irregularly 

 branching and anastomosing as they approach the periphery, 

 and sending out at right angles minute, flexuous, calcigerous 

 tubes into the large, irregular clear interspace ; the enamel-like 

 surface forms a wide, dark-coloured band of fine, straight, pa- 

 rallel calcigerous tubes at the circumference of the magnified 

 section. 



The above generic name brings to mind not only the pecu- 

 liarly rugged crag-like aspect of those teeth, but also their re- 

 lation to the genera Orodus and Acrodus (Ag.), from both of 

 which they are distinguished by a simply conic form, the base 

 being nearly circular and not greatly lengthened in one direction 

 as in those genera, as well as the dense stony character of the 

 surface and great depth, coarseness and star-like disposition 

 of the superficial vertical ridges. The base is slightly hol- 

 lowed in the middle below, of a coarsely osseous texture, pene- 

 trated with numerous small vascular canals. It is interesting to 

 observe that of all the fossil teeth yet made known, the micro- 

 scopic structure of the Petrodi approaches nearest to the internal 

 characters of the recent Cestracion. 



Petrodus patelliformis (M'Coy) . 



Sp. Char. Conical, height one-half to two-thirds the width of the 

 base, which is round or rarely subtrigonal ; apex rudely pointed, 

 becoming flat by wear; sides radiatingly ridged with about 



