ADDENDA. 483 



8tro[)liodonta j^eniculata. 



Plate XXIII. Fia. 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 strire, 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 liniestoues of the age of the Lower Hel- 

 derberg group : Cumberland, Maryland; 



RtlYNCnoSPIRA. 



A farther examination of the fossils designated in the volume as a 

 subgenus of Trematobpira, has convinced me of the propriety of separating 

 them as a distinct genus, which has already been thus published in the 

 Regents' Repoi't on the State Cabinet of Natural History for 1858, p. 29( 



