128 INTRODUCTION. 



Crania. The second pair of sliding muscles, by which the dorsal valve is restored to its 

 place, is described as being attached at the points marked r, r. I failed in attempting 

 to determine these, and also in ascertaining the points of attachment of the muscles of the 

 peduncle. 



Obs. Several authors, among v\rhom wq may mention Dr. Gray,^ Professors King^ and 

 M'Coy,'' have satisfactorily shown that the term Orbicula, having been apphed by Cuvier, 

 in 1798,* and subsequently likewise by Lamarck,^ to Patella anomala of Muller, it ought 

 not to be retained for the shells under description, as the species described by the Danish 

 naturalist belongs to the genus Crania!' 



For many years I felt unwilling to discard the term Orhiculdf on account of its 

 currency, occasioned by Professor Owen's description of the animal of Discina (0.) 

 lamellosa / but as the term Orhicida in the Cuvierian and Lamarckian sense is merely a 

 synonym of Crania, we are obliged to adopt the genus Discina!' proposed and described 

 by Lamarck, in 1819, and typified by D. ostreoides, a shell no doubt identical with 

 0. lamellosa of Brodrip, but which never lived on the recent maritime shores of Great 



^ Annals of Philosophy, new series, vol. x, 1825; and An. and Mag. of Nat. Hist., vol. ii, 2d series, 

 p. 4.39. 



2 A Monograph of Permian Fossils, p. 84, 1849. 



3 British Pal. Fossils in the Camb. Mus., 1852. 



* Tableau Elementaire du Regne Animal. " On ne connait de ce genre qu'tme seule espece decouverte 

 par Muller en Danemarch, et ranffee jusquici parmi les Patelles ; Patella anomala." In 1802, the same 

 author states that Lamarck had adopted his views. 



^ Annales du Museum, vol. x, 1807. In that memoir, Lamarck places Crania in the family of 

 Oysters; Orbicula, Terebratula, and Lingula, in the Brachiopoda. In 1801, the same author describes 

 the genus Orbicula, founded on O. Norvegica = Patella anomala of Muller, and in 1819, distinctly states 

 that it was not attached by a pedicle, but by the substance of the lower valve. 



^ The Baron de Ryckholt figures in his ('Melanges Paleontologiques,' pi. iv, fig. 3) specimens of 

 Crania obsoleta, under the appellation of Orbiculoidea Cimacensis, said to be from the Devonian of Cbimay 

 and Ferque, in which the shell is bordered by a fringe, which he terms " impressions ciliaires," and 

 regards these as the remains of the horny setts which surround the margin of the mantle of the recent 

 animal, and which are also developed to a less extent in Rhynchonella. Not having seen the original 

 specimens, I cannot offer an opinion, as to whether these beautiful examples really exhibit the fossilised 

 border of the mantle, or only the fringed margin of the shell itself. 



"^ In Parts I and III, I made use of the term Orbicula in the general sense, but not in that of Cuvier 

 and Lamarck ; the term Discina, will have to be there introduced, and Orbicula erased. Professor E. Forbes 

 informs me, that "it would certainly be more convenient, under all circumstances, to prevent confusion, 

 to use Discina instead of Orbicula, and to suppress altogether the latter name." 



^ Trans, of the Zool. Soc, vol. i, 2d part, 1833 (printed in 1835) ; and in the Annales des Sciences 

 Nat., 1845. 



" "Discina; coquille incquivalve, ovale-arrondie un pen deprimie ; a valves de grandeur 6gale, 

 nyani chacune un disque orbiculaire central tr^s-distincte . Disque de la valve supirieure nan perce, ayant 

 au milieu une protuberance en mamelon ; celui de V autre valve tris-blanc, divis6 par unefente transversale." 

 Lamarck, 'Hist. Nat. des Animaux sans Vertebres,' vol. vi, p. 236, 1819. 



