CONCHIFEEA. 



235 



much accuracy as do the forms and processes of fossil bones 

 that of the vertebrate animals to which they belonged. 



As our limits prevent us from giving a detailed description 

 of the different parts of this calcareous envelope, we refer the 

 student to the plate of the shells and description thereof, 

 and to Sowerby's Illustrated Conchological Manual, for 

 details on this part of the subject, and likewise for a descrip- 

 tion of recent and fossil genera. 



In the grouping of this class the adductor muscles were 

 regarded by Lamarck as affording a primary character ; hence 

 his division into Monomyaria and Dimyaria, according as 

 they had one or two muscular imprints in each valve. 



The foot is a muscular organ which most conchifera 

 possess. It is employed for digging, leaping, and swimming, 

 and other locomotive acts ; and its form varies in the dif- 

 ferent genera. The following table of the families, by 

 Deshayes, will assist the student to understand the value of 

 the characters on which he has grouped this class : 



DIMYARIA 



CONCHIFERA 



MONOMYARIA I Having a foot . 

 L Having no foot . 



f TUBICOL*. 

 PHOLADARI&. 

 OSTEODESMATA. 

 MACTRACEJE. 

 TELLINIDES. 



CoNCHjE. 



CYCLADES. 



CARDIACE.E. 



LTRIDACNE.B. 



TNAYADE8. 



The lobes of the mantle I TRIGONE.E. 

 disunited . . . | ARCACE^E. 

 (^ 



f The lobes of the mantle 

 united in a greater 

 or less degree, poste- 

 riorly .... 



f MALLEACE.S:. 

 | PECTINIDES . 



OSTRACEJE. 



The shells of the conchifera are found in the most ancient 

 rocks ; their generic forms have slightly varied during the 

 long periods of time which have elapsed between the primary 

 epoch in which they commenced life, and the modern seas 

 where they so greatly abound. Most of the genera which 

 lived in the ancient periods exist at the present time, al- 

 though the species has changed from one period to another. 



Nothing is observed in the history of this class to justify 



