On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 441 
much flatter and more nearly pentagonal in outline. Their surface is worn into deep 
hollows seemingly by impact with the rounded teeth of the opposing jaw. In a 
few cases the thick dentine is completely worn through, exposing the bony structure 
below. The base in the larger specimens is quite half an inch in thickness. The 
smaller teeth, of which there are numerous examples, appear to have been attached 
to the antero-lateral extremity of the larger ones, they are broadest at the posterior 
sutural margin and gradually diminish in size anteriorly, where they are connected 
with a third anterior tooth which is trigonal in form, ending in front with an 
obtusely pointed angular margin (PL LV,, figs. 1, 2,3). It is not probable that more 
than three teeth existed on each side of the jaw and the teeth hitherto known as 
Helodus plunus, Ag., occupied the intervening portion of the palate. An example 
(PL. LYV., fig. 6) shows a Aelodoid tooth to have occupied a position in front of the 
anterior tooth as well asthe palatal space behind. 
Formation and locality : Mountain Limestone, Armagh: Kendal. 
Ex coll. Karl of Enniskillen. 
Genus.—Pecilodus, Agass., MSS. 
Palatal teeth, length equal to half the breath; subtrapezoidal in outline. 
Crown, surface raised into folds extending radially from the posterior basal 
extremity ; transverse lines of ridges extend parallel to the posterior margin 
decreasing in size forwards. Posterior margin sigmoidally sinuous ; postero-lateral 
margin more or less straight ; antero-lateral margin convex or straight, converging 
forwards. Anterior margin, narrow, convoluted downwavrds. 
Median tooth narrow, triangular in outline. 
The genus Peecilodus was instituted by Professor Agassiz (Poiss. Foss., Wel, JOUL,, 
p. 174) and included six species, viz., P.jonesti and P. transversus, P. angustus and 
P. obliquus, P. sublevis and P. parallelus, of these more recent discoveries have proved 
the two latter species to be teeth from the jaw of the same fish and that they differ 
in character and form from the typical P. jonesii so completely, that in 1858 
Professor Agassiz removed them from the genus Peecilodus and made them the type 
of a new genus which he named Deltodus: ?. parallelus, being the median tooth of 
P. sublevis. The two species P. jonesii and P. transversus, were also proved to 
belong to the same fish and the latter has been merged into the former. | 
Professor M‘Coy (Brit. Palzeoz. Foss., p.638), described the genus Peecilodus and 
in addition to the Agassizian species P. jonesii (with P. transversus), P. obliquus, 
P. parallelus and P. sublevis described two new species, P. aliformis and P. fovertatus. 
The last, from the Carboniferous Limestone of Derbyshire, appears to possess the 
characters of the genus whilst P. aljformis is without doubt a Deltodus and must be 
relegated to that genus. The description of the genus as given by Professor M‘Coy 
is in consequence of the separation of the teeth to form the genus Deltodus, 
inappropriate and insufficient ; the one above is therefore substituted. ,,, 
