100 ROYAL SOCIETY OF CANADA 
Thus the anterior adductors, two of the most important muscles 
of the Brachiopoda are wanting or obscure in Hyolithes and their func- 
tions are performed by the laterals. 
VASCULAR SYSTEM. 
A cast of the inner surface of a large operculum figured by Holm 
(Fig. 5) shows a remarkable sculpturing such as the author has not 
seen elsewhere.’ 
It represents what might be considered a vascular trunk some dis- 
tance within the margin of the horizontal limb of the operculum, from 
which apparent trunk numerous branches are directed toward the 
posterior margin of this operculum. 
It is common to find narrow ribs or rays diverging from the apex 
of the cone between the two sets of muscles, and rarely to find those 
rays branching, but no pseudo-trunk such as Dr. Holm represent has 
been observed. - In ordinary cases observed by the author, these rays 
after passing out from behind the lateral muscle, curve more or less 
forward, and while some reach others do not attain the posterior margin 
of the operculum. It seems quite probable that they mark the position 
of the tubes of a vascular system. 
Similarly on the crown of the cone and extending forward from it, 
one finds on the inside of the operculum a series of radiating ridges, 
traceable part of the way and in some cases all the way to the front 
margin of the operculum. As these rays are quite distinct on the 
very apex of the cone in the operculum, there seems no possibility of the 
existence of a true umbonal muscle in Hyolithes. These rays at the 
front of the valve at least, are very like in appearance to the median 
vascular grooves in the Brachiopoda. No branches from these trunks 
appear to flow around outside of the imprint of the lateral muscles, and 
backward toward the hinge, as in the Brachiopoda. In Hyolithes then 
we seem to have a circulatory system in front of the apex as in the 
Brachiopoda, but also another system of vascular passages behind it. 
The narrowness of the posterior border of the dorsal valve in Acro- 
thyra will not admit of comparison with corresponding portion of the 
opercule in Hyolithes, but a parallel to the method of growth in this 
opercule may be found in Orbiculoidea and allied genera. 
On the whole then it would seem that the resemblance between 
’ Acrothyra and Hyolithes is to a considerable degree superficial, and 
that the latter genus differs radically in some important structural 


1Sverig. Kamb-Silur. Hyolith. och Conul., tafl. 5, fig. 47. 

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