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BULLETIN OF THE NATURAL HISTORY SOCIETY. 
concentrated around the forarainal passage, and the shell appears to 
have been of sedentary habit, since the ventral valve in many cases 
is found fossil in such an attitude as to show that this valve stood in a 
vertical position in the mud of the sea bottom when the animal which 
inhabited it was living, the opening of the valve being uppermost. 
No such uniformity of attitude characterizes the dorsal valve. 
We find that the ventral valve in Acrothyra assumes quite a 
different attitude. It lies in almost all cases on its side, and usually 
with the opening of the valve uppermost. Moreover, it is to be noted 
that on successive layers these valves lie with the umbo oriented in a 
fixed direction. From this it may be inferred that they give evidence 
herein of the action of a current, flowing in a definite direction and 
sweeping the valves in the direction towards which the current set. 
They may have swung in this direction by the pedicle while the 
animal was living ; or when swept away by the flowing water, have 
presented the point of least resistance to the current, as they sank to 
the bottom. In either case we must regard Acrothyra as living under 
different conditions from Acrotreta, which, as we have remarked, 
apparently had the apex of the ventral valve buried in the mud. 
It is in accordance with these conditions that we have in Acrothyra 
a visceral callus developed along the median line of the ventral valve, 
as is the case in Lingula and other allied genera; and Lingula, as is 
well known, had a long pedicle. 
This genus is peculiarly Etcheminian, there being two species and 
several varieties or mutations in the strata of this age. It seems likely 
Linguldla (?) inflata of the Protolenus Fauna belongs to Acrothyra; 
if so, the genus ranges up into the base of the Cambiian. 
Conotreta, of Walcott an Ordovician (Trenton) genus, is a later 
development from the Acrotretoid phylum, differing in the form of 
the visceral callus, which is pointed in front, in place of expanding, 
as in Acrothyra. Analogy, however, would lead us to infer that this 
genus also was free-floating, and not sedentary, like many species of 
Acrotreta. 
This type of Brachiopod — Acrothyra — is one of the earliest known 
in the Palaeozoic rocks of Canada, being found in shaly layers in the 
midst of the eruptives which mark the advent of Pala?ozoic Time in 
Eastern North America. 
