38G 
BULLETIN OF THE NATURAL HISTORY SOCIETY. 
This mutation shows a change in the direction of A. 'proavia 
described below, and of a higher horizon.. 
The ventral valves lie on their sides on the layers of the rock, but 
the dorsals are on edge. 
Acrothyra proavia. pi. XIV, figs. 2 a-g and 3 a-f. 
Acrotreta j^yoavia, n. sp. Xat. Hist. Soc. N. B. Bull., Vol. iv, p. 203, 
pi. iii, figs. 2 rt to / 
Shell substance calcareo-corneous. The thin outer crust sometimes 
wanting from corrosion, abrasion or absorption. 
Ventral valve oblique-conical, with a prolonged beak. Caidinal 
area narrow, as is also the pseudo deltidium ; in the pseudo deltidium 
near the apex is a small oval tubercle, between which and the apex, the 
foramen is supposed to be situated. The valves slope evenly down 
from the apex to the anterior and lateral margins. Xo good examples 
of the interior of this valve have been obtained ; imperfect ones show 
two vascular lines enclosing a narrow visceral callus, and extending as 
far down from the apex on one side as the hinge area does on the other; 
the front margin exhibits on the interior a row of about ten radiating 
vascular ridges. This valve is often undulate with one, sometimes 
several strong grooves concentric to the umbo, marking periods of rest 
in the growth of the shell ; corresponding ridges are found on the 
deltidial area. 
The dorsal valve is round, and broadly rounded in front ; the con- 
tour of the surface is varied by a moderate projection of the umbo 
behind, and by a slight flattening of the valve in front, giving the 
valve a round, slightly triangular relief. The interior has the impres- 
sion of a pair of muscles in the umbo, whence a low ridge extends for- 
ward across the valve. Not infrequently the edges of this valve are 
flattened, and one or more grooves, concentric to the umbo, marking 
stages of growth, indent it. 
Sculpture . — A irtrong lens reveals a series of concentric striie on 
the surface on some examples of this shell, there being about 20 in 
the space of a millemetre. Between these ridges a still stronger 
magnifier (1 inch objective) shows a fine granulated surface with 
occasional rows of coarser granules, parallel to the concentric striie. 
On the inner, chitinous surface there is a similar ornamentation, but 
less distinct than that on the surface of the outer layer. 
