518 On the Fossil Fishes of the Carboniferous Limestone Series of Great Brituin. 
keep the cutting edge of the tooth sharp. Descending from the apex of the convex 
anterior coronal surface there is in some cases a closely approximating series of 
open ducts or canalicule, they are about an eighth of an inch in length and extend 
with a slightly sinuous line and not unfrequently bifurcate. 
The general resemblance of some of the lower teeth of Petalorhynchus to those 
named by Professor Agassiz, Chomatodus truncatus (Pois. Foss. vol. iii. p. 174), 
and afterwards described by Professor M‘Coy, asa Petalodont form of Chomatodus 
(Brit. Palseoz. Foss. p. 618, pl. 31, fig. 1), strongly suggests the probability that 
the two genera may have appertained to the same fish. They are described by 
Professor M‘Coyas being “longitudinally oblong, sides of crown and root sub-parallel, 
except at the upper and lower ends, when they rather abruptly converge to form 
the subtruncate, slightly rounded cutting edge at top, and the rather more pointed 
lower end of the root ; crown flattened, bent at an angle of 120° with the long root 
which is nearly straight in profile; surface of the crown finely punctured as in 
Psammodus ; root, hard and polished, with two or three obscure lonsitudinal 
furrows, almost entirely surrounded by a raised marginal extension of the thick, 
prominent, simple, coronal ridge ; the posterior side 1s concave. Length six and 
a half lines, width six lines, length of crown two lines.” 
Count Miinster has described a series of teeth of Janassa (Beitrage zur Petrefac- 
tenkunde, Heft V. p. 38, tab. 15, fig. 10-14), which appear in all more important 
particulars to have possessed great similarity in arrangement to those of Petalo- 
rhynchus. In Janassa the terminal teeth are similar in form to certain Petalodonts, 
and it may perhaps be assumed that the teeth hitherto known as Chomatodus 
truncatus, were the terminal teeth of Petalorhynchus. They differ considerably 
in form, but the teeth which are undoubtedly Petalorhynchus, bridge over the 
difference between the sharp, long pointed form of the central teeth and those, 
even and flat crowned, which occupied each posterior extremity of the jaw. 
Awaiting evidence to the contrary they are here included with Petalorhynchus. 
Three species of Petalorhynchus are described from the upper beds of the St. 
Louis Limestone, of Illinois (Palzeont.of Hlinois, vol. VI. p. 405-409, pl. XIL., figs. 1-8). 
P. pseudosagittatus St. J. and W. is described as an example of the intimate specific 
relations existing between the European and American Carboniferous fishes. It 
very nearly approaches P. sagittatus Agass., from the Limestone of Armagh, but 
“is distinguished by its shorter base, less rapidly converging outline of the crest, 
and the more numerous imbrications of the coronal belt.” Considering the extreme 
variety in the form, size, and characteristics which exist amongst the numerous 
specimens from Armagh, it is slightly problematical if the above differences are 
sufficient to constitute a separate species. The same remarks may apply to P. 
distortus, St. J. and W., itis possibly a lateral tooth of P. pseudosagittatus, the twist 
in the crown being due to its position in the mouth. The third species P. spatu- 
jatus is principaliy characterized and distinguished by its broadly expanded base, 
