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 palseozoic " 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, iil. vii, figs. 10-12. 1866. 

 J British Palaeozoic Fossils, p. 190. 



