20 FISH OF THE CARBONIFEROUS PERIOD. 



or four imbricating folds of ganoine, irregularly jagged at 

 their edges. 



This species is rather smaller, more elevated and more com- 

 pressed than the C. ductus (Ag.), from which it is further distin- 

 guished by the number of minor cones set on the mesial ridge. 



Rare, in the red carboniferous limestone of Armagh. 



{Col. University of Cambridge.) 



Petalodus rhombus (M'Coy) . 



Sp. Char. Crown sharp, compressed; anterior face rather more 

 than three times wider than high, regularly rhombic, the upper 

 and lower margins almost symmetrical, the lateral portions of 

 each being nearly straight or slightly concave, and meeting in 

 the middle at an obtuse angle ; the angles of the upper and 

 lower margins nearly equal, that of the basal margin pointing 

 down, of the cutting edge up ; the cutting edge is obscurely 

 undulato-dentate by short obsolete vertical furrows, and mi- 

 nutely crenulato- striated; the lower margin is prominent and 

 surrounded by five or six small imbrications of pseudo-enamel 

 or ganoine, those descend lower (as usual) on the posterior 

 face, where also the mesial angulation is less ; surface smooth 

 with a few fine irregular longitudinal strise ; root moderately 

 large. Height of the middle of the crown 6 lines, width about 

 1 inch 9 lines, depth of root about 7 lines. 



This can only be confounded with the P. acuminatus (Ag.), 

 from which it is fully distinguished by the great proportional 

 width of the crown and the rhomboidal form of the anterior face 

 produced by the similar shape of the superior and basal margins ; 

 the latter being deeply angulated in the middle, resembling the 

 cutting margin reversed in the present species, while the middle 

 half is very obtusely rounded in the former, and then abruptly 

 curved upwards and again downwards further towards the ends ; 

 in that species also the cutting edge, although slightly waving, 

 is destitute of the broad longitudinal plicse of the present form. 



Rare, in the carboniferous limestone of Derbyshire. 



(Col. Cambridge University.) 



Polyrhizodus (M'Coy), n. g. 



Gen. Char. Tooth thick, crown but slightly elevated, forming a 

 transversely oval crushing surface, narrowing towards the ex- 

 tremities ; anterior and posterior ridges separating the crown 

 from the root, obtuse, without imbricating folds, the posterior 

 ridge much lower than the anterior ; root very large, deeply 

 divided into several distinct, root-like lobes or fangs. 

 This is perhaps the most remarkable genus of ichthyolites yet 



