Notes on Cincinnatian Fossil Types 
289 
That the concave curvature and stellate grooving of the 
exterior surface of the plates is not confined to Maysville forms is 
indicated by a specimen (1914, Bulletin, Denison Univ., 17, pi. 4, 
fig. 4j, found a short distance beneath the Fulton clay shale, contain- 
ing the characteristic Triarthrus hecki, in the upper part of the Point 
Pleasant beds of Orton, at the quarry located a quarter of a mile 
west of Point Pleasant, Ohio. In this specimen, the division lines - 
between the plates are indicated by sharply impressed narrow 
grooves. The plates are hexagonal and concave; and distinctly 
impressed and relatively broad grooves extend from the angles 
of the plates toward the center, where they unite. The plates, 
apparently, must have been thin and practically there is no room 
for a concavity within the individual plates. 
This structure evidently is quite distinct from that of Pasceolus 
halli Billings, the first described and first illustrated species of the 
genus, ranging from the Gamachian series, younger than the Rich- 
mond, into the Silurian strata of Anticosti. In this species, the 
exterior surface of all of the plates is distinctly convex. The plates 
probably are hollow, their lower surface is convex, and a short spine 
extends from the center of the base of each plate toward the center 
of the specimen. The short spine may be seen distinctly in speci- 
mens of Pasceolus gregarius Billings, a smaller sized species occurring 
in the Silurian strata of Anticosti after the disappearance of typical 
Pasceolus halli. 
Apparently there are two groups of species included in Pasceolus^ 
the group typified by Pasceolus halli, with convex plates, and the 
group typified by Pasceolus darwini, with concave plates and stel- 
late grooves. The former usually are strongly calcareous. The 
latter occur in strongly argillaceous strata. The former occasionally 
preserve very delicate surface markings. In the latter no surface 
markings ever have been noticed in addition to the stellate grooves 
already described. This has raised the question whether it might 
not be possible that the so-called plates of the darwini group of 
Pasceolus in reality represent only the basal parts of the plates, 
the upper and lateral parts having been removed by weathering. 
In that case the stellate grooves would be features characterizing 
that part of the base of the plate which faces the hollow in the 
interior of the plate; and the grooves would converge toward the 
point of departure of the spine. I have not been able to either 
prove or disprove this hypothesis, with the specimens at my command. 
