LOWER HELDERBERG ROCKS. 
8 59 
Genus DlSCIM (Lamarck). 
Discina : Lamarck, 1819, Hist. Nat. des Animaux sans vertebres. 
Orbicula of authors, not of Cuvier and Lamarck. 
The fossil shells usually referred to Orbicula by authors, are very 
properly arranged, by Dr. J. E. Gray, Prof. Fleming, Prof. King, Prof. 
M‘Coy, and Mr. Davidson, under the Genus Discina , a name proposed by 
Lamarck for a recent species, the Discina ( Orbicula) lamellosa of authors. 
The term Orbicula was originally used by Cuvier and Lamarck to de¬ 
signate a recent species, which is known to belong to the Genus Crania*. 
The following are some of the exterior characters of the shells of this 
genus, as given by Mr. Davidson : 
“ Circular, longitudinally or transversely oval. Upper or dorsal valve 
conical, patelliform, with the apex inclining towards the posterior 
margin. Lower or ventral valve opercular, flat or partly convex, and 
perforated by a narrow oval longitudinal slit, reaching to near the 
posterior margin, and placed in the middle of an oval depressed disk. 
“ Surface smooth, ornamented by numerous striae radiating from the 
apex to the margin, or by concentric lines of growth produced in fo- 
liaceous expansions. Shell structure horny, and perforated by minute 
tubuli.” 
Discina discus (n. s.). 
Plate IX. Fig. 13 - 15. 
Compare Distinct (Orbicula) forbesii, Davidson, Bull. Soc. Geol. France, Deuxiemc S6rie, Tome v, 
pa. 334, pi. 3, f. 45. 
Memoirs Geol. Survey of Great Britain, Vol.ii, Part i, pa. 321, pi. 26, f. 2. 
Shell orbicular, rarely subelliptical in form. Upper or dorsal valve mo¬ 
derately convex : apex nearly central. Lower or ventral valve nearly 
* See : Davidson, Introduction to the Classification of the Brachiopoda. 
Dr. Gray, Annals of Philosophy, New Series, Vol. x, 1825; Ann. and Mag. Nat. History, 
Vol.ii, Second Series, p.349. 
Prof. King, Monograph of Permian Fossils, p. 84, 1849. 
M'Coy. British Pal. Fossils in Cambridge Museum, 1852, p.,190, etc. 
