COirCHIFEEA. 405 



'cke shells conld not be determined from sncli specimens. Our 

 knowledge of the ex:tinct Radiolite is derived from natural 

 moulds of the interior, formed before the dissolution of the 

 inner layer of shell, or from specimens in which this layer is 

 replaced by spar. 



The necessities of geologists have compelled them to pay very 

 minute attention to the markings in the interior of shells, to 

 their microscopic texture, and every other available source of 

 comparison and distinction. It must not, however, be expected 

 that the entire structure and affinities of molluscous animals 

 can be predicated from the examination of an internal mould oT 

 a morsel of shell, any more than that the form and habits of an 

 extinct quadruped can be inferred from a solitary tooth or the 

 fragment of a bone.* 



The systematic arrangement of the bivalves now employed is 

 essentially that of Lamarck, modified, however, by many recent 

 observations. The families follow each other according to rela- 

 tionship, and not according to absolute rank; the Veyuridce are 

 the highest organised, and from this culminating point the 

 stream of affinities takes two courses, one towards the My as, 

 the other in the direction of the oysters ; groups analogically 

 related to the Tunicaries and Brachiopoda. 



SECTION A. AsiPHOKiDA. 

 a. Pallial line simple : Integro-pallialia, 



Fam, 1. Ostreidse. t 4. Arcadse. 



2. Aviculidse. 5. Trigoniadse* 



3. Mytilidse. { 6, TJnionidse. 



SECTION B. SiPHONiD'A. 



7. Chamidse. 



8. Hippuritidse. 



9. Tridacnidse. 

 10. Cardiadse. 



11, Lucinidse. 



12, Cycladidse* 



13, Cyprinidse. 



h. Pallial line simuded : Sinu-pallialia, 



14. Yeneridse. 



15. Mactridse. 



16. Tellinidse. 



17. Solenidse. 



The characters which have been most relied on for distin- 



18. Myacidse, 



19. Anatinidse. 



20. Gastrochsenidse. 



21. Pholadidae. 



* Eludes Critiques sur les Mollusqiies Fossiles.par L. Ar/assiz, Neuchatel, 1840. 



