DISCINID. 73 
some, but probably only minor, characters which separate it from the ordinary Orbicule 
(Discine) ; the shell is generally more solid and calcareous ; both valves are nearly. equally 
convex, and the passage for the muscle of attachment, instead of beimg through a /ongi- 
tudinal fissure as in Orbicula (Discina), is considerably contracted, being confined to a 
small tubular perforation situated at the marginal end of a rather deep closed furrow. 
The pedunculated form assumed by the muscle of attachment must have allowed greater 
freedom of motion to the animal, and may be the reason for the more conical develop- 
ment of the lower valve in this genus, as distinguished from the compressed form of the 
same valve in Ordicula.” He also adds that Dr. Kutorga was certainly mistaken when 
placing Schizotreta in his family of Stphonotretee. 
I entirely coincide with Prof. Morris im the above remarks; but, as no interiors of 
Orbiculoidea have yet been discovered, it has not been possible to compare its muscular 
impressions with those of Discina. It may be desirable to maintain provisionally 
D’Orbigny’s subgeneric designation for the forms possessing the characters above 
described. 
O. Forbesti is the only British species we can at present with certainty refer to 
D’Orbigny’s subgenus. In his ‘Prodrome’ Discina Morrisii is referred to the same 
group, but I do not think there is sufficient evidence for this. 
OrsicuLoipEs Forsesi, Dav., 1848. Pl. VII, figs. 14—18. 
OrspicuLta Forsestt, Dav. Bull. Soc. Géol. France, 2nd ser., vol. v, p. 334, pl. iii, 
fig. 45, May, 1848. 
— — Phillips and Salter. Memoirs of the Geological Survey of 
Great Britain, vol. ii, Part I; Paleeontological Appendix, p. 
371, pl. xxvi, fig. 2, 1848. 
ORBICULOIDEA — Dav. D’Orbigny, Prodrome de Paléontologie Stratigraphique, 
vol. i, p. 44, 1849. 
— — Morris. Ann. and Mag. of Nat. Hist., 2nd ser., vol. iv, p. 321, 
pl. vii, fig. 3, 1849. 
DiscIna ~- Morris. Catalogue, p. 134, 1854. 
= — Salter. Siluria, 2nd ed., p. 250, fig. 11, 1859. 
Spec. Char. Shell longitudinally oval, rather narrower posteriorly; both valves convex ; 
the upper or free one most so, conoidal or limpet-like, being more or less elevated; the 
apex is subcentral, but varies in position between the centre and the posterior margin ; 
the external surface is regularly marked with concentric ridges or lines of growth. 
The opposite or perforated valve has its apex also subcentral, and sometimes near the 
posterior margin. ‘The small circular perforation is placed close to the marginal end 
of a longitudinal and rather deep sulcus, which varies in length at different ages and in 
different specimens. ‘The external surface is regularly marked with narrow concentric 
10 
