[MATTHEW] CAMBRIAN FAUNAS 95 
The amount of chitinous matter in the valves of the species of 
this group is small, and the outer shell may have been calcareous. As 
preserved in the shales of the Cape Breton Cambrian, it is trans- 
lucent and apparently silicious. These shells may be related to 
Obolella. 
OgoLus LENS (Plate 1, fig. 6). 
This form is considerably larger than the preceding, and, like it, 
has regular concentric ridges on the surface. In this character they 
differ from the Oboli of the Lower Etcheminian, which have a surface 
ornamentation of waving irregular ridges. 
OBozus BRETONENSIS (Plate 1, fig. 5). 
The preceding species is found both above and below this one; 
the former is found in fine sandy shale, but the present species affected 
a mud in which clay was more plentiful. This species had more 
prominent and more widely set concentric ridges on the surface than 
the preceding. 
These three forms belong to the section PALÆOBOLUS, char- 
acterized by approximated vascular trunks. 
In the base of this division of the Etcheminian is a large Obolus, 
whose characters are not sufficiently known to enable us to use it 
in this comparison (O? major). 
OBOLUS PULCHER (Plate 1, fig. 7). 
In the base of the St. John group another Obolus appears, of a 
type quite different from either of the preceding. It is easily recog- 
nized by the peculiar ornamentation of cancellated ridges, resembling 
those of Zphidea pannula. The dorsal, by its incurved and flattened 
posterior slope, and its internal markings, shows a resemblance to the 
markings of Acrothele, and the beak of the ventral is more prominent 
than is usual in Obolus; but it is not pushed forward as in Acrothele, 
nor is the cardinal area visible from above. 
One peculiar feature of this species is the larger size and fan- 
like form of the callus of the ventral valve; in this point it resembles 
some of the Orthids. The callus extends beyond the middle of the 
valve, and shows that in the ventral valve of this species, the central 
muscles were far forward. This is referred to the subgenus Bors- 
FORDIA. 
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