FISH OF THE CARBONIFEROUS PERIOD. 21 



made known, as it presents the only instance in the entire class 

 of a fish- tooth divided into several distinct fangs — the imperfectly 

 double, divaricating, base of certain sharks^ teeth, or the pro- 

 longed external pliccC of certain sauroids [HoloptychiuSf Rhizodus, 

 &c.) not deserving to be viewed in the light of really divided or 

 fanged roots as in the mammals and the present genus of fish. 

 The group is most allied to Petalodus (Owen), with which Agassiz 

 seems to have unaccountably confounded one of the species ; but 

 instead of the thin, scale-like or petal-like character of the Peta- 

 lodiy with their highly elevated, compressed, sharp-edged crown, 

 we have here a tooth of remarkably thick and clumsy form, with 

 the crown but little raised and all the parts obtuse ; the crown in 

 Petalodus is covered with a thick coat of smooth, highly polished 

 enamel-like substance or ganoine, and separated on each side 

 from the root by several imbricating folds, while the crown in the 

 present genus is nearly as dull as the root, and in one species 

 coarsely punctured as in Psammodus, and the characteristic basal 

 imbrications are replaced by a simple, obtuse ridge ; finally, in 

 Petalodus, the root forms a large, simple, compressed truncated 

 base, while in Polyrhizodus it is divided into from five to eight 

 fangs. 



Polyrhizodus magnus (M^Coy). 



Syn. Petalodus radicans (Ag. MSS. name in collections). 



Sp. Char. Tooth from one-half to two inches wide and about one 

 inch in depth ; crown from three to four lines wide, nearly flat, 

 inclined at an angle of about 70° from the slightly raised pos- 

 terior edge, smooth, or with minute branching strise on the 

 upper edge ; anterior and upper margins nearly parallel for the 

 middle half of the width, while they rapidly converge in the 

 two outer fourths, so that the extremities are narrowed to a 

 point at each end and considerably bent downwards ; root very 

 thick and deeply divided into six or eight long, ovate fang- 

 like lobes, roughened by the passage of the nutrient vessels ; 

 inferior, posterior ridge is (like the anterior margin of the 

 crown) simply rounded, without imbricating folds, and about 

 one-third more than the width of the crown below the cutting 

 edge. 



This fine species is not uncommon in the limestone of Armagh. 

 [Col. University of Cambridge, Capt. Jones, Mr. Griflith, &c.) 



Polyrhizodus pusillus (M'Coy). 



Sp. Char. Crown compressed, raised into a rounded obtuse lobe 

 less than half the width of the base in height ; base of the 

 crown forming a very prominent obtusely rounded ridge ; root 

 divided into about ten small rounded fangs ; surface of the 



