LOWER HELDERBERG ROCKS. 159 



Genus Discdja ( Lamaeck). 



Discina : Lamarck, 1819, Hist. Nat. des Animaux sans vertebres. 

 Orbicuta of authors, not of CuviBR and Lamarck. 



The fossil shells usually referred to Orhicula by authors, are very 

 properly arranged, by Dr. J. E. Gray, Prof. Fleming, Prof. King, Prof. 

 M'CoT, 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 Cuviee and Lamaeck 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. 

 " SuKFACE 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. Fia. 13 - 15. 



Compare Discina {Orbicula) forbesii, Davidson, Bull. Soc. G4o1. France, Deuxiemc S^rie, Tome v, 



pa. 334, pi. 3, f. 45. 



McmoirB GeoL Survey of Great Britain, Vol.ii, Tart i, pa. 321, pL 20, 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 tlie Bracbiopoda. 



Dr. Gray, Annals of Philosophy, New Series, Vol. x, 1825; Ann. and Mag. Mat. History, 



Vol.ii, Second Series, p. 349. 

 Prof. KiKO, Monograph of Permian Fossils, p. 84, 1849. 

 M'CoT, British Pal. Fossils in Cambridge Museum, 1852, p. 190, etc. 



