[MATTHEW ] ACROTHYRA AND HYOLITHES 103 
718 mm. in size, angle or aperture 1 to 2°6. The place on the dorsal 
side which in a brachiopodous shell is marked by the callus,in this has 
two faint diverging striæ; these begin near the apex and extend 5 mm. 
from it; the shell there is nearly 3 mm. wide, and the area between the 
striæ, corresponding to the Brachiopod callus is about one-half of this 
width. A smaller example, 12 mm. long, has a visible stria on one side 
only of the tube. 
The reverse of the first-named tube measures 7X19 mm., angle 
of aperture 1 to 2-7. On the interior of this (dorsal side) one vascular 
line extends 6 mm. from the apex. Three diaphragms are faintly 
marked in the substance of the tube in the first 2 mm. of its length; here 
the shell is more rigid than near the aperture. This valve has sharply 
ridged margins along the two edges on the dorsal side, showing a thick- 
ening of the shell there. - 
HYOLITHES SERICEUS (Horizon 14, Porter’s Brook). 
Some large examples, one tube 1138 mm.; angle of aperture 1 
to 3°5. These show the sculpture on both sides, but do not show any 
further characters for comparison with Acrothyra. 
HYOLITHES SERICEUS, OPERCULE. Long. diam. 8 mm.; transv. diam. 
84mm. Loc. cit. figs. 7¢ & 751 (Horizon 14), Interior. The “cone” or 
central area near the apex is marked by diverging ridges; those of the 
middle run about one-third of the distance to the front margin; some 
near the border of the cone run about half-way to the front of the 
opercule. 
The ridges (radiating somewhat on the broad chevron outside of 
the cone) run nearly to the margin of the opercule; these are supposed 
to mark the line of advance of the lateral muscles as the shell grew. 
Outside the chevron, 1.e., behind it is a narrow furrow showing a few 
radiating ridges. 
Some examples of the opercule show a broad, low median swelling 
in the middle of the cone, and an actual median ridge behind the apex. 
The opercule is very thin, and the apex is one-eighth or more from the 
posterior margin. 
The posterior or horizontal limb of the opercule is marked by about 
5 slightly curved ridges on each side running to the margin; they curve 
forward slightly in going outward; there is one ridge stronger than the 
others on each side of the median line, curved more directly backward ; 
they would meet the grooves on each side of the median area on the 
ventral side of the tube; between these two ridges the posterior limb of 
the opereule is somewhat elevated; this part would correspond to the 
deltidium of a brachiopodous dorsal .valve. 
Se TIRE Tia à 

1 See also fig. 4 of this article. 
