212 UPPER, OR FLINTY CHALK. 



In the third vol. of Mr. Parkinson's " Organic Remains," a fragment 

 of an inoceramus is accurately represented, (PI. v. fig. 3), and is noticed 

 by that author as " part of an uncommon fossil shell, resembUng a pateUa 

 in some of its characters*." 



The celebrated geologists, M. M. Cuvier and Brongniart, were in- 

 duced from the striated crystalline structure of the shells in question, to 

 regard them as belonging to the genus Pinna; but fragments of the hinge 

 having been subsequently presented to their examination, they were led 

 to doubt the correctness of their former opinion f. 



For a knowledge of the structure of these curious shells, we are in- 

 debted to the acumen and zeal of Miss Benett, of Norton House. From 

 specimens in her cabinet, and with the assistance of others collected by 

 the present writer, Mr. Sowerby has constructed the following generic de- 

 finition, 



Inoceramus. 



A free, inequilateral, beaked bivalve ; hinge linear, channelled, trans- 

 versely sulcated, extending on one side of the beaks only ; cartilage partly 

 external ; no visible muscular impression ? 



The shells of this genus are more or less gibbous, and are commonly 

 marked with transverse concentric ridges, and striae ; their constituent sub- 

 stance is invariably composed of crystallized carbonate of hme, of a radiated 

 or fibrous structure. 



* Organic Remains, Vol. iii. p. 5L 



t " II n'est pas sur que les gros fragmens planes, de 12 millemetres d'epaisseur, et a tex- 

 ture striee qu'on trouve dans la craie, appartient a ce genre de coquille. Nous avons vu chez 

 M. Defrance, des portion de charniere qui indiquent un autre genre." Geograjph. Min. des 

 Enviro?is de Pai-is. p. 11. 



Perfect specimens of this fragile tribe of shells are so exceedingly rare, that even at the 

 present time, the distinguished naturalists above mentioned, have not been able to obtain an 

 example, in which the structure of the hinge is satisfactorily shewn. 



In a recent communication from M. Brongniart, that gentleman remarks, " Je suis ex- 

 tremement embarasse pour determiner la coquille fibreuse si commune dans la craie, et dont 

 vous avez represente diverses parties dans votre Planche xxvii. J'ai voulu aussi faire figurer 

 cette coquille, mais n'ayant pas eu le bonheur d'en avoir des echantillons assez entiers, j'ai ete 

 force de me contentir de fragments, et de donner la copie de votre figure 1, PI. xxvii." 



