392 
BULLETIN OF THE NATURAL HISTORY SOCIETY. 
visceral callus is of a circular form, and only one-quarter of the length 
of the valves. Its ridge closely encircles a deep pit, which lies just 
in front of the foraminal opening, in the direction of which it becomes 
narrower and shallower. The traces of a pair of straight diverging 
grooves are discernable at the sides of the callus. 
This consists of minute concentric ridges, visible only 
with a strong lens. 
Size. — Length, 2 mm.; breadth, 2J mm.; depth, 1 mm. 
Horizon and locality. — Fine gray shales in the volcanic beds at 
Dugald Brook, Escasonie (C. B.), X. S. Scarce. 
The short callus distinguishes this species from A. crothyra signata- 
prima with which it occurs. The pit in this callus, though so short, is 
analogous to that of A. signata, so that in this earliest fauna these two 
types of umbonal muscle scar and groove were already diflferentiated. 
Acrotreta c. f. sociALis. vonSeebach. PL XV, figs. 5 a-k. 
c. f. Acrotreta socia/is, v.Seeb., Zeitschr. der Deutsch. geol. Gesell- 
schft., Vol. xvii, 1865, p. 341, pi. viii, a, figs. 1-4. 
c. f. xicrotreta socialis, v.Seeb., G. Linnarsson, 1896, Brachiopods of 
Paradoxides Beds of Sweden, Bihang till K. Svenska vet. Akad- 
Handl. Bd. 3 Xo. 12, p. 16, Tail, hi, figs. 32-34. 
A small species with coarse surface characters and strong muscle 
scars. 
Ventr<d valve moderately elevated, sub-circular in outline, some- 
what flattened on the cardinal slope, where, in outline, it is slightly 
convex ; nearly straight on the anterior slope. Interior — At the back 
there are one or two faint grooves on the median part of the cardinal 
slope ; the foraminal boss is a wedge-shaped one with the point 
directed forward ; this is enclosed by two sub-parallel, deep, rounded 
pits, for attachment of muscles. Behind the foramen is the back 
of a ridge, similar to a crescent, that encloses the apical part of 
the shell behind, and laterally ; in the front half of the space thus 
enclosed is a faint outline of a visceral callus of a lozenge shape. 
The position of the vascular trunks is probably outside of the horns 
of the crescent, thence extending forward ; about a third or a quarter 
from the front of the shell is a crescentic row of short vascular- 
grooves. In front of this row of grooves are one or two growth 
ridges, and the flattened border of the valve. 
