ADDENDA. 
483 
Strop hod out a geniculata. 
Plate XXIII. Fig. 6 a, b, c. 
Shell somewhat semicircular, abruptly geniculate towards the base. Dor^ 
sal valve flat for two-thirds the length of the shell, when it is abruptly 
inflected. Ventral valve with a narrow area, the beak projecting a little 
beyond the opposite, slightly convex from the beak towards the middle, 
becoming flattened and concave, and the last third abruptly deflected, 
corresponding with the opposite valve. 
Surface, at and near the beaks, marked by strong radiating striae, which 
bifurcate, and are increased by interstitial additions towards the base 
of the shell. Tubular openings are noticed at intervals on the summits 
of the stronger striae, and round or oval pores mark the interstitial 
spaces. 
In some specimens, the centre of the dorsal valve is marked by a strong 
sinus with a prominent elevation on each side, with a corresponding 
mesial elevation on the centre of the opposite or concave valve, and a 
sinuosity on either side. 
Fig. 6 a, b. Ventral and dorsal views of a specimen of medium or small size. 
Fig. 6 c. Profile view of the same. 
Geological position and locality. In the limestones of the age of the Lower Hei¬ 
delberg group : Cumberland, Maryland* 
RHYNCHOSPIRA, 
A farther examination of the fossils designated in the Volume as a 
subgenus of Trematospira, has convinced me of the propriety of separating 
them as a distinct genus, which has already been thus published in the 
Regents’ Report on the State Cabinet of Natural History for 1858, p. 29* 
