[MATTHEW | CAMBRIAN FAUNAS 97 
as the type, is evidently open to question; the arrangement of the 
muscle scars and of the vascular trunks, relates them to other genera 
rather than to Obolus, as typified by the species above named. For 
this reason we have felt it necessary to propose sub-generic names 
to indicate these important differences. 
The oldest group (Hoobolus) differs from the type in the 
advanced position of the whole group of the central muscles of the 
dorsal valve; not the “ 7”? laterals alone. 
The second group Lol differs in the approximation of 
the vascular trunks of the ventral valve, which enclose only a third 
of the area of the valve, whereas in O. Apollonis one-half of the 
surface is thus enclosed. 
The third group (Botsfordia) differs in the close grouping of 
the central scars of the dorsal valve, and the posterior position of 
the lateral scars. Also in the very large callus of the ventral valve. 
The fourth group represented by Obolus pristinus is different 
by the arrangement of the scars from any of the preceding, and also 
by its sculpture. It is, perhaps, on the line of development of Obolus 
Ella which Mr. Walcott, on account of its surface, has referred to 
his subgenus Westonia; it, however, has no resemblance to Westonia 
in its surface-markings, and the “ j ” Jaterals of the dorsal are much 
farther forward. 
The fifth group (Monobolina) differs from the type in the close 
approximation of the anterior adductors of the dorsal valve. 
There are also differences in the sculpture In the first group 
(Eoobolus) this consists of close, fine irregular waving concentric ridges. 
In the second (Palæobolus), of stronger and more regularly concentric 
ridges. In the third (Botsfordia), of cancellated ridges, forming a 
marked pattern. In the fourth, of fine concentric ridges, irregular 
in their elevation and prominence. In the fifth (Monobolina), in 
very fine and regular concentric ridges. 
The following table is an attempt to show graphically the possible 
relations of the several species to each other, and to other genera, 
and subgenera of the Cambrian system. 
The table shows the horizon in the Cambrian at which each of 
the species appeared in Eastern Canada, with some vieuiclas cE to 
other species occurring outside that region. 
