t5ULLEtIN OP THP: NAtUKAL HISTORY SOCIETY. 
For information regarding several European species of Acrotrata I 
am indebted to Mr. Gilbert Van Ingen, of the School of Mines, Col. 
umbia College, New York. 
ACROTHELE Linnarsson. 
Acrothele avia. pi. xvi. Figs, la to f and 2« and 6. 
Acrothele avia, n. sp. Nat Hist. Soc. Bull., vol iv., p. *202, pl. iii., 
figs, a to h. 
A rather large species with oblately oval valves and a thick horny 
shell. 
Ventral valve somewhat concave in front of the apex. This valve 
has a triangular, somewhat convex, high area, including a narrow, 
slightly convex, pseudo deltidium, divided into two equal parts by an 
obscure central groove. There appears to be a foraminal opening at 
the slender pointed apex. 
The interior of the ventral valve is marked by a shallow circular 
pit, on each side of the pedicle opening ; and behind, at the margin, 
is a shallow triangular pit, resembling the pedicle groove of an Obolus 
In front of the pedicle opening is a strong oval tubercle, on each side 
of which extend the ridges that bound the oval centre of the visceral 
cavity. Four low vascular ridges extend forward from this oval area 
to the anterior margin of the valve. The parts of the interior of the 
shell, above described, are enclosed by an ox-bow shaped groove, re- 
.sembling the impression of vascular trunks ; these trunks have about 
four anterior branches, and each trunk extends nearly to the front of 
the valve. On the lateral slopes of the shell are three crescentic 
grooves, which may be accidental and due to pressure. 
The dorsal valve is strongly bent down behind and in front. The 
umbo is slightly prominent, is appressed, and is close to the posterior 
margin. The sides of this valve have about a dozen radiating, branch- 
ing, crenulated ridges, that extend to the margin. 
The interior of the dorsal valve has a strong median septum, ex- 
tending to the middle of the valve ; at the end of this ridge is the 
central muscle. Scars of the latter are seen on each .side of the broad 
(;nd of the median l idge, near its end. On each side of the median 
ridge at the cardinal margin are two larger pairs of muscle .scar.s. 
