1858.] WOOD RED CEAG. 33 



To ascertain the age of the Red Crag by the "percentage test," 

 could only be effected by the exclusion of a large portion of its con- 

 tents ; and, when we have got rid of those fossils which we know to 

 belong to some remote anterior period, we should then have to con- 

 tend against the doubtful admission of many species that may have 

 been introduced from the deposits immediately antecedent. A large 

 number of the shells have in all probability, as suggested by Sir 

 Charles Lyell, been washed into the Eed Crag Sea by an underwater 

 abrasion of the older beds ; and the shells by such action may have 

 suffered little or no deterioration during the removal, and when 

 redeposited would have nothing in their appearance to indicate their 

 derivative nature, nor would their specific character help us to de- 

 cide whether they were natives or foreigners ; and that is probably 

 the condition of many of the specimens found in this fossil museum 

 of natural curiosities. 



The following genera may have supplied species to the Red Crag, 

 namely, Glimna, Cardita, Astarte, Cyprina, Isocardia, lAmopsiSy 

 Turritella, Vermetus, Cancellaria, Pleurotoma, Terehra, Valuta, and 

 Pyrula. All of these have been obtained from the disturbed portion 

 of the Red Crag ; and none of them are found in the genuine de- 

 posit at Walton-on-the-IS'aze, which I consider the type of the Red 

 Crag. Out of 240 species of mollusca found in the Red Crag, and 

 belonging truly to a Modem Tertiary period, fifty may perhaps be 

 considered as derivative fossils. 



The following species found in the Red Crag may be enumerated 

 as having been possibly derived from the Coralline Crag. 



Voluta Lamberti. Mytilus hesperianus. 



Cypraea affinis. Chama gryphoides. 



Erato Maugeriae. Diplodonta dilatata. 



Terebra inversa. Lucinopsis Lajonkaireana. 



Cassidaria bicatenata. Cardita senilis. 



Nassa conglobata *. chamaeformis. 



labiosa. — ^ — orbicularis. 



prismatica. scalaris. 



Trophon alveolatum. Astarte Basterotii. 



consoeiale. Burtini. 



Pleurotoma carinata. gracilis. 



intorta*. incrassata. 



semicolon ? mutabilis. 



CanceUaria mitraeformis. Omalii. 



scalaroides. Isocardia cor. 



Scalaria foliacea, Cyprina rustica. 



varicosa. Venus casina?. 



Vermetus intortus. ovata?. 



Dentalium costatum. Circe minima. 



Tapes texturata*. 



Ostrea princeps. virginea ? 



Hinnites Cortesyi. Mactra glauca *. 



Pecten dubius. obtruncata. 



maximus?. Panopsea Faujasii. 



Pinna pectinata. Mya truncata?. 



Limopsis aurita. Grlycimeris angusta. 



* These have not yet been found in the Coralline Crag, but they probably 

 belonged to the period of that deposit. 



VOL. XV. PART I. J) 



