[MATTHEW ACROTHYRA AND HYOLITHES 107 

EXPLANATION OF FIGURES. 
1. Acrothyra proavia. mut. Mag. —a, Ventral valve, cardinal view—b, side 
view of same—c, Dorsal view of a mould of the interior—d, Dorsal valve seen 
from above. All from the Upper Etcheminian fauna of Cape Breton. 
2. Hyolithes obesus, Holm. Natural size—a, Mould of interior of the dorsal 
side of the tube (the lower part of the mould is broken away). From the 
Paradoxides Œlandicus zone, Sweden—b, Same species, ventral side of the 
tube. From Horizon id, P. Tessini Zone of the St. John Group, N. Brunswick, 
Canada. ; 
3. Hyolithes costatus. Mag. ?—a, The tube, ventral view—b, Operculum, 
seen from above. From Upper Cambrian, Mt. Stephen, Br. Columbia. 
4, Hyolithes sericeus. Mag. —Operculum, seen from above; to show the 
vascular ridges of the horizontal limb, etc. Horizon 1d St. John Gr. 
5. Hyolithes Œlandicus, Holm. Operculum, Mag. 3, after G. Holm (sketch), 
to show what appear to be radiations from vascular trunks on each side 
toward the margin of the horizontal limb of the operculum. From the P. 
Œlandicus zone, Sweden. 
N.B.—Compare 1a with 2b and 3a. Compare ie with 2a. Compare 1d with 
3b, 4 and 5. 
Though these species of Hyolithes are convex along the middle area of the 
ventral side, other species are concave along that part. Also many brachio- 
pods have a convex deltidium or pseudodeltidium, where this Acrothyra is 
concave. 
