BRACHIOPODA. 
129 
The Orbicula Morrisi was described by Davidson* in 1848 from imperforate 
valves only, and his subsequent noticef of the species (1866) as (?) Discina 
Morrisi, did not add essentially to the knowledge of its characters. In 1855, how¬ 
ever, McCoy had noticed^ the species as Discina Morrisi, and, without giving any 
figures, described the pedicle-valve, from which it would appear that it is de¬ 
pressed-convex, concave about the margins, the “ fissure ” being “ very broad, 
oval, half its length from the posterior margin.” We are not able to gather 
from these meager details, or from any other published description of the species, 
the precise character of the foraminal aperture, and having been unable to 
obtain suitably preserved specimens of the pedicle-valve, can not directly com¬ 
pare this feature with that of the forms under discussion above. However, the 
imperforate valve in the species is strongly convex, with an excentric, poste¬ 
riorly directed apex, and this fact combined with McCoy’s description of the 
pedicle-valve, gives to the species a general contour in precise harmony with 
that prevailing among the palaeozoic “ Discinas ” generally, while those species 
now passing under the name of Orbiculoidea, Davidson, are distinctive in hav¬ 
ing the relative convexity of the valves reversed, the pedicle-valve being the 
more convex. 
This relative convexity of the valves in “ Discina,” though always liable 
to diminution from compression in fossilization, is a permanent feature, and, 
as far as our observations extend, invariably associated with the foraminal 
characters already described, except in the instances cited below. We there¬ 
fore feel justified in regarding all these species as congeneric (with reservations 
in favor of forms of the type of Orbiculoidea conica, 0. Forbesi, etc.), and adopting 
for them the term, Orbiculoidea, D’Orbigny, not Davidson, = Discina (palae¬ 
ozoic species), of authors. 
Diagnosis. Shells subcircular or subelliptical in outline, inequivalve. Apices 
excentric. Pedicle-valve depressed-convex or flattened, with the apex slightly 
elevated and inclined posteriorly. On the exterior of the valve a narrow pedi- 
* Bull. Soc. Geol. de France, Second Series, vol. v, p. 334. 1848. 
t Silurian Brachiopoda, p. 65, .pi. vii, figs. 10-12. 1866. 
t British Palaeozoic Fossils, p. 190. 
