[MATTHEW] ACROTHYRA AND HYOLITHES 97 
that figured by G. Holm,’ for Hyolithes obesus of the Paradoxides 
Œlandicus zone (—to P. lamellatus zone, Div. 1¢ at St. John), a species 
which occurs also in the Paradoxides beds at St. John (Div. 14), but 
at a somewhat higher horizon ; this also has vascular rays near the 
apex on the interior of the dorsal side. 
But in both these cases there is this distinction from the callus of 
Acrothyra—the pseudo-callus of Hyolithes fades away into the gen- 
eral smooth.surface of the interior of the tube, and does not exhibit 
the abrupt frontal termination shown in the callus of the ventral valve 
of Acrothyra. The presumption is therefore that Hyolithes had no 
muscles attached to the tube, like the anterior abductors in brachiopods; 
or if any such existed they were weak. 
Several of the earlier species of Hyolithes show a median convex 
area, on the ventral side of the tube, analogous to the pseudo-deltidium 
of Acrothyra, and of about the same average breadth; and this is limited 
on each side by a sharp furrow, or beaded groove. At this line, on the 
ventral side, the striz of growth that cross it change somewhat in their 
course as those of Acrothyra and Lingulella do. Hyolithes Hathewayi 
of the Protolenus Fauna in Newfoundland? in which the striæ arch 
backward in approaching the median area and on the latter become 
transverse, is striated like Acrothyra. 
Another point of comparison between the ventral valve ‘of Acro- 
thyra and the tube of Hyolithes is the comparative thickness and texture 
of this part of the exo-skeleton in the two genera. As in other Brach- 
iopods the shell of Acrothyra shows a graded thickness from the visceral 
callus through the visceral area to the brachial space. So also as was 
shown in H. Hathewayi, there are three conditions of the body wall on 
the dorsal side of the tube in going from the apex to the lip. By the 
crushing of the tubes of Hyolithes in shaly layers, which often occurs, 
a similar condition of the shell is betrayed by the proximal end shown in 
relief, the middle part marked by irregular longitudinal ridges, and the 
distal part of the tube smoothly flattened.® 
THE OPERCULUM. 
As the tube in Hyolithes compares with the ventral valve in Acro- 
thyra, so the operculum resembles the dorsal valve. 
The subcircular form of this valve in Acrothyra and Acrotreta and 
the subtriangular form of the elevated part of the valve, are strikingly 

1Sverig. Kamb-Silur. Hyolith. och Conular. Stockholm, 1893. Tafl. 5, 
fig. 48. 
2Trans. Roy. Soc. Can., 2nd ser., vol. V., p. 73, pl. iii, fig. 5 a and b. 
8 See also the tube of Hyolithes gracillimus figured on a later page. 
