LINN^AN ARRANGEMENT. 21 



shells, as tlie periosteum does to the bones, and is called the 

 perioste by Adanson ; but how it contributes to their growth 

 does not appear clear, as all shells seem to increase by succes- 

 sive layers of stony matter on the inside of the shell, which 

 is secreted from some part of the animal, each layer projecting 

 a little at the edges beyond the preceding ones : the edges of 

 these layers may generally be perceived with the naked eye ; 

 they oftentimes form the lines which in the Bivalves are com- 

 monly denominated transverse strice (^pl. 2. f. 20). 



The Bivalves, which are divided by Linnaeus into fourteen 

 genera, are distinguished by some peculiarity of the hinge, 

 principally by the number, form, or sitviation of the teeth, 

 which are small protuberances found near to the beak or car- 

 tilage. 



The Univalves are divided by Liiniaeus into eighteen genera, 

 and are distinguished by sometliing peculiar in the forma- 

 tion of the mouth or aperture of the shell. The generic cha- 

 racters are given by Linnaeus as follow: but if they were 

 strictly adhered to, it is vei-y evident that several new genera 

 must be introduced to receive many shells which could not 

 properly be included in any of these. 



MULTIVALVES. 



Chiton. Shell of many pieces longitudinally on the back. 

 JLepas. Shell of many pieces unequal and sitting. 

 Pholas. Shell of two pieces with several smaller beliind. 



