FISHES OF THE CARBONIFEROUS SYSTEM. 209 
tumid, irregular, projecting but slightly below the arched margin of the 
anterior face. 
These teeth have somewhat the aspect of those of A. cucullus,’ N. & W., 
when seen from the posterior side, but they are less high, the band of enamel 
folds at the base is much broader, and they are more numerous; while the 
anterior face, with its upper half strongly arched and its single deep sulcus, 
proves it quite distinct. 
Among the species described in the report cited below, A. robustus and 
A, similis (PI. II, Figs. 9, 10) are much like this, but neither has the char- 
acteristic dorsal sulcus of the present species. The differences noticeable 
are also increased in significance by the fact that all the species compared 
come from different beds of the Carboniferous limestone, as A. robustus from 
the Chester beds, A. similis from the Keokuk, while the species now 
described is from the Saint Louis limestone at Spencer, Owen County, Ind. 
Potyruizopus Lirronr, N. & W. 
Plate XIX, Figs. 5-6. 
A short description and a single view of a tooth belonging to the above 
species have been published.’ I have received from the Saint Louis lime- 
stone at Greencastle, Ind., teeth of the same species which show an interest- 
ing variation of form from that which was taken as the type. One of these 
has been figured to show this difference. The root, it will be seen, is much 
longer in proportion to the crown, and the concave surface occupies not 
much more than half of the posterior face. These differences may be indi- 
vidual, but more likely are dependent upon different positions in the mouth 
of the fish. Living sharks and the few fossil ones of which anything like 
a complete dental series has been obtained show considerable diversity of 
both size and form among the teeth which occupied central and lateral posi- 
tions in the mouth, and frequently a marked difference between those of 
the upper and lower jaws. Hence there is danger that among fossil teeth 
which were scattered broadcast over the sea bottom such differences will 
! Geol. Survey Illinois, vol. 2, p. 41, pl. 3, fig. 1. 
*Tbid., vol. 4, p. 357, pl. 4, figs. 10, 10°. 
MON XvI-—14 
