ADDENDA, 
487 
( amurium typum (n. s.). 
Plate XCV A. Fig. 2 a <$• b, 3, 5 & 6. 
Shell short-oval or subelliptical, length but little greater than the width, 
very ventricose or subglobose. Ventral valve much the larger, extremely 
arcuate, the curvature from beak to base being somewhat more than 
half a circle; most gibbous on the umbo and near the front, which is 
bent abruptly upwards, and produced into a broad rounded extension : 
beak incurved, thin and pointed; umbonal slopes sharply angular, sub¬ 
parallel to the cardinal margin, with a moderately broad, smooth, and 
slightly concave space between; foramen large, triangular, reaching 
nearly to the extremity of the beak. Dorsal valve most gibbous on the 
umbo, elevated in front into a prominent mesial fold; the sides curved 
downwards, and produced to meet the receding edges of the opposite 
valve : beak rather large, incurved beneath the opposite beak, and 
apparently filling its foramen. 
The interior of the ventral valve has a deep large rostral cavity, and a 
more or less highly arched transverse septum, which, rising from be¬ 
neath the rostral cavity, extends to near the middle of the valve, and 
reaches about two-thirds across its transverse diameter. From the bases 
of the dental plates, rise two diverging thickened elevated ridges, 
which extend to the sides of the arching septum, and, uniting with it, 
gradually die out upon its surface. 
Surface of the shell, when sufficiently well preserved, bears evidence of 
faint radiating striae. 
Fig. 2 a. Dorsal view of Camarium typum. 
Fig. 2 b. Profile view of the same, 
Fig. 3. Interior of the ventral valve, showing the transverse arching septum. 
Fig. 5. A longitudinal section of fig. 3. 
Fig. 6. A cast of the interior of a valve similar to fig. 3. 
Geological position and locality. In rocks of the age of the Lower Helderberg 
group : Cumberland, Maryland. 
