68 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 16, 
tween several genera. Several species of Trematis have been already 
published under the name of Orbicule, all of which are found in 
Lower Silurian beds. 
The genus may be defined as follows :— 
TREMATIS ; a suborbicular, inequivalve Brachiopod attached by a 
ligament passing through a longitudinal fissure in the posterior part 
of the ventral valve. Valves united by a hinge which is supposed to 
resemble that of Terebratula, and is accompanied in the dorsal valve 
by three diverging internal plates. Shell regularly punctated ex- 
ternally ; pearly, fibrous, and slightly striated internally. 
1. TREMATIS TERMINALIS. 
Orbicula terminalis, Emmons, Report on the Geology of New York, 
pt. 2. p. 395. f.4; Hall, Paleeontology of New York, pl. 30. f. 11. 
Shell subquadrate, rounded, broader than long, depressed ; both 
the valves are slightly convex, but the convexity of the lower valve 
is interrupted by a large depression sloping towards the fissure, which 
reaches from the centre of the shell to near the hinge ; surface smooth, 
marked with regular punctations distinctly visible to the naked eye. 
Breadth half an inch, length ;4ths of an inch. 
Found in the “‘ Trenton limestone”’ of New York and in the * Blue 
limestone” of Ohio: the specimen figured was sent to Mr. Lyell by 
Mr. Clark, who found it in the “ Blue limestone” at Cincinnati. 
Upper valve. 
Trematis terminalis. Lower valve magnified. 
2. TREMATIS CANCELLATA. 
Orbicula cancellata, G. B. Sowerby, Zoological Journal, vol. ii. 
ph bd. £6. 
Shell orbicular, very flat, beg more gibbous near the posterior 
extremity: surface covered with close-set elevated lines radiating 
from the apex, which are crossed by the elevated lines of growth, so 
