Pisces. | UPPER PALA®OZOIC VERTEBRATA. 637 
very prominent middle portion, or shaft, of the spear-head, produced by the peculiar curvatures of the coronal 
bands ; the coronal bands of the P. psittacinus being simply transverse, or slightly arched. 
Position and Locality—Not very uncommon in the red lower limestone of Armagh. 
Explanation of Figures.—PI. 3. 1. fig. 2, front view of old much worn tooth, natural size, shewing the very 
long root, the truncation of the apex by wear, and the unusual puncturing of the surface from the same cause; 
fig. 8, ditto, view of the inner side of a less worn specimen, shewing that the transverse folds at the base of 
crown are more numerous and much lower in position on the inner than on the outer part; part of the root 
broken off; fig. 3 a, front view of same tooth; 3 4, profile of the same tooth; 3c, ditto, part of cutting edge 
magnified. 
Genus. PETRODUS (J/‘Coy). 
Ref—MCoy, Ann. and Mag. Nat. Hist. 2nd Series, Vol. IT. 
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. Jicro- 
scopic structure: vertical and horizontal sections nearly similar, shewing the centre to be composed of exceed- 
ingly 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, parallel, calcigerous tubes at the 
circumference of the magnified sections. 
The above generic name brings to mind not only the peculiarly rugged crag-like aspect of those teeth, 
but also their relation to the genera Ovodus 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 hollowed in the middle below, of a coarsely osseous 
texture, penetrated with numerous small vascular canals. It is interesting to observe that of all the fossil 
teeth yet made known the microscopic structure of the Petredi approaches nearest to the internal characters 
of the recent Cestracion. 
PETRODUS PATELLIFORMIS (JA/°Coy). PI. 3. G. figs. 6, 7, 8. 
Ref—M Coy, Ann. and Mag. Nat. Hist. 2nd Series, Vol. IT. 
Desc.—Conical, height one-half to two-thirds the width of the base, which is round or rarely sub- 
trigonal; apex rudely pointed, becoming flat by wear; sides radiatingly ridged with about thirteen or fourteen 
very strong, single or dichotomous ridges, the sides of which are usually cut by numerous deep oblique sulci; 
the ridges are highest at the base, where they terminate abruptly; osseous base a little wider than the crown. 
Diameter of base three to four lines. 
This tooth presents considerable variations in the proportions of height to width of the base, and also in 
the number and relative thickness and complexity of the ridges; there is no variety however sufficiently striking 
to require particular notice, or occasion any difficulty in the identification of the species. 
It seems abundant in some parts of the Derbyshire limestone. 
Explanation of Figures.—Pl. 3. G, fig. 6, single perfect tooth, viewed in profile, natural size; fig. 6a, 
ditto, magnified ; 64, vertical section from grinding surface to base, highly magnified, shewing the numerous, 
irregularly branching, medullary canals, giving off dentine tubes at right angles, and the superficial coat of 
dense ganoine, without canals; fig. 7, tooth, natural size, seen from above, the apex worn down by use; fig. 8, 
tooth, natural size, under side of base. 
