252 MANUAL OF THE MOLLUSCA. 



\ 



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

 spar. i 



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 or a | 

 morsel of shell, any more than that the form and habits of an extinct quad- 

 ruped can be inferred from a solitary tooth or the fragment of a bone.* j 



The s)/stematic arrangement of the bivalves now employed is essentially i 

 that of Lamarck, modified, however, by many recent observations. The j 

 families follow each other according to relationship, and not according to i 

 absolute rank; the Venerida are the highest organized, and from this culmi- ; 

 nating point the stream of affinities takes two courses, one towards the Myas, 

 the other in the direction of the oysters ; groups analogically related to the 

 Tunicaries and Brachiopoda. ,, 



SECTION A. AsiPHONiDA. 

 a. Pallial line simple : Integro-pallialia. 

 Fam. 1. Ostreidfe. 4. Arcadee. 



2. Aviculidaj. 5. Trigoniada;. 



3. Mytilidae. 6. Unionidaj. 



SECTION B. SiPHONTDA. 



11. Lucinidse. 



12. Cycladidse. 



13. Cyprinidse. 



7. Cbamidfe. 



8. Hippuritidae. 



9. Tridacnidse. 

 10. Cardiadse. 



b. Pallial line sinuated: Sinu-pallialia, 



18. Myacidae. 



19. Anatiuidse. j 



20. Gastrochaenidae. \ 



21. Pholadida. j 



14. Veneridse. 



15. Mactridse. 



16. Tellinidfe. 



17. Solenidse. 

 The characters which have been most relied on for distinguishing these 



"Toups and the genera of bivalves are the following, stated nearly in 'the 

 order of their value: — 



1. Extent to which the mantle-lobes are united. 



2. Number and position of muscular impressions. 



3. Presence or absence of z. pallial sinus. 



4. Form of the foot. 



5. Structure of the branchice. 



* Etvdes Critiques sur les Mollusques Fossiles, par L. Agassiz, Neuchatel, 1840. 



