96 ROYAL SOCIETY OF CANADA 
And further it may be observed that theoretically this would be the 
natural position if Hyolithes arose from a slender camerated or chamber- 
less membraneous tube of a worm which in that condition sought shelter 
and food amid the debris on the sea-floor, and before the acquirement of a 
firmer and larger tube had enabled it to compete with its fellows and its 
enemies in the clearer water above the muddy bottom. This position 
then, with the orifice of the tube upward, appears to be the one which 
Hyolithes would naturally assume. 
To compare Acrothyra with this genus it is also necessary to place 
its ventral valve with the opening upward? and orient it so that the car- 
dina! area will correspond to the ventral side of the tube in Hyolithes ; 
the anterior and lateral slopes of the valve will then correspond to the 
dorsal side of the tube in Hyolithes, and the-curving front of the valve 
to the dorsal lip of the genus named. (See figs. le and 2a.) For a 
parallel to the pedicle of Acrothyra, there remains the larval tube 
(usually hypothetical in the adult of Hyolithes). In many of the early 
forms of Hyolithes the parallel of the cardinal area of the brachiopod 
is found in the three areas often outlined on the ventral side of tubes 
of Hyolithes. (See figs. la, 2b and 3a.) 
Nor is it in the ventral valve alone that point of agreement with 
Hyolithes can be found, for the dorsals also have a general resemblance 
to the opercule in Hyolithes that will be shown more in detail in the 
sequel. 
The species of Hyolithes used in this comparison were, for the 
Acadian provinces, those of the Paradoxides-Abenacus zone (Div. 1d), 
(equivalent in its fauna to the P. Tessini zone of Sweden) for Sweden 
those figured and described by Dr. G. Holm,” and for those of Bohemia 
the species of Barrande’s classical work, the “Silurian System in 
Bohemia.” 
THE TUBE OF HABITATION. 
One of the dominant parts in the structural features of the 
interior of the ventral valve in Acrothyra is the visceral callus, the 
indicator of the position and growth of the anterior adductor and other 
central muscles. It is difficult to find anything analogous to this in 
Hyolithes ; nevertheless we can point to two cases where something 
similar appears in the tube of this genus. One is a tube of Hyolithes 
sericeus from the Paradoxides beds at St. John, in which near the apex 
on the dorsal side, vascular ridges outline a space corresponding in posi- 
tion to the callus in Acrothyra. Another case of such a pseudo-callus is 
1 See group of figures at the end of this article. 
2 Swedish Cambrian-Silurian Hyolithidæ and Conulariidæ. Stockholm, 
1893. 

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