382 
BULLETIN OF THE NATURAL HISTORY SOCIETY. 
shell, where it is supposed, to connect with the foramen. Outside the- 
callus, on each side, near the margin of the valve, are lenticular marks 
of the lateral muscles. About the middle of the valve the position of 
the anterior adductors is indicated by a faint impression of the lozenge 
or “ heart-shaped ” depression. The margin of the valve is thickened.. 
The dorsal valve is orbicular, strongly convex, and has a somewhat 
triangular appearance, because of the sides being depressed from the 
umbo, and because the front is strongly bent downward. Interior . — 
This shows a sharp, thin, median septum for half of the length of the 
valve. This ridge is broader and more distinct at the front ; at each 
side are lateral obscure ridges, diverging from the umbo. Outside 
of these ridges are the lenticular imprints of the lateral muscles. 
Sculpture . — The surface of this shell (which perhaps is not the 
real outer surface) is shining, and has fine concentric ridges visible 
only with a lens. 
Size. — Length of the ventral valve, 3 mm.; width, 2 mm.; depth, 
1 mm. In the dorsal the length and breadth are equal, and the depth 
is less than that of the ventral. 
Horizon and locality. — This species' is found in Assise h of the 
lowest Etcheminian Zone at Dugald Brook, a branch of Indian Brook 
in Escasonie, X. S. 
On a cursory examination the ventral valve of this shell might 
pass for that of a Lingulella, but the closed deltidial area is that of 
Acrotreta and Acrothyra ; the form of the dorsal and the nature of 
its interior show that the species is closely related to Acrotreta. 
Acrothyra signata-prima. Plate XIII, figs. 1 a-g. 
Test (calcareo-)corneous ; valves tumid. Ventral valve variable in- 
form, longer than broad, often quite tumid, with the posterior half 
straighter than the anterior, which in some examples is strongly 
arched down toward the margin. Hinge area variable in height, beak 
sometimes overhanging the hinge, sometimes withdrawn from the per- 
pendicular. Interior — There is a visceral callus from one-quarter to 
one-third of the length of the valve, wider in front than behind, bordered 
Vjy vascular grooves ; the central depression is del^per toward the apex 
than toward the front. Outside of the aforesaid grooves is another and 
a shorter pair, more widely diverging ; traces of the lateral muscle scars 
are seen outside of this latter pair of grooves. 
