100 ROYAL SOCIETY OF CANADA 
it is conceivable that if we could follow the series in the phylum 
of Acrotreta farther back in time, we would meet valves in which 
the umbo would be no higher than in Acrothele or Linnarssonia; or 
even until the umbo came down to the plane of the edges of the valve. 
The fourth column represents the area of the opening of the 
valve, assuming it to be square, which, of course, it is not; but the 
extra third is left as an offset to the “third dimension,” viz., the 
height, which is not shown in this and the following tables. This 
column is intended, therefore, to represent the bulk of the shell. 
In the case of Acrotreta it may be gathered from this table that the 
Ordovician Acrotretas had attained three times the bulk of the 
species that appeared at the beginning of Cambrian time. This then 
is the result of the observations already made on the genus Acrotreta. 
Let me see how far this result is borne out by observations on 
other genera. 
Several genera of Brachiopods are known to have appeared 
simultaneously in the base of the Cambrian in the Acadian provinces 
of Canada. These are: 
Acrothyra. 
Acrotreta. 
Leptobolus. 
Lingulepis. 
Lingulella. 
Obolus. 
On comparing the species of these genera which appeared in the 
Coldbrook or lowest terrane, with the species in the next terrane 
which they most resemble, a decided, though not very great increase 
in bulk, is observable. This will be seen by the following table: 






COLDBROOK TERRANE ETCHEMIN TERRANE 
£|£ =| 8 
SPECIES AND Mutations | #|m | § SPECIES = real esa 
2 ee ov CA I 
wile ld SE 4] 4 
mmimm mm|mm 
Acrothyra signata-prima....| 2 | 24 | 4°50|| A. signata........ PAR MG: E.1b 
Acrotreta, papillata prima..| 2 | 24 | 5‘00 | A. papillata....... 24 | 24 | 625 |E.3d 
Leptobolus, torrentis........ 30|20146:00! Wa atavus "22 5 | 34 11750 | E.3e 
Lingulepis pumila ........ .| 6 | 44 | 27° L. longinervis ....|11 | 8 |88° |E.2b 
Linguella, c.f. longovalis....| 74 | 5 |37°5 || L. longovalis..... 916 |54 |E.le 

Obolus torrentis............. 6 |6 |36- O. triparilis.... .. O18) 726 Hake 
