396 MOLLUSCOUS ANIMALS. 



and when it is nearly abortive, we know the animal to be either fixed to 

 his place, or enabled to move by swimming. Dr. J. E. Gray has divi- 

 ded the Lamellibranchiata into orders according to the nature of the 

 the foot, but though deserving of much attention, it is very doubtful 

 whether it marks progress of development as well as the characters 

 which we have previously considered, and it has the practical disadvan- 

 tage of not being observable without seeing the animal alive, or in a well 

 preserved condition. The medium of respiration, whether fresh or salt 

 water, is deserving of much attention, and the microscopic structure of 

 the shells is of considerable importance, there being a manifest progress 

 observable from the lower to the higher forms, the painted porcellanous 

 form seeming to claim a higher position than the plainer solid form ; 

 that than the nacreous, and all of these above the succession of thin 

 overlying plates. Other characters, as slight peculiarities in the branchiae, 

 position of the ligament, dentition of the hinge, &c., seem to be of so 

 much less importance, that though they may be advantageously used for 

 genera and families, they cannot be supposed to furnish distinctions of 

 orders. -Jt must nevertheless be observed that considerable reduction in 

 the size of the shell-bearing portion of the mantle, accompanied by the ' 

 habit of burrowing in stone, clay or wood, and frequently by the forma- 

 tion of a shelly tube to protect the Siphons, forms a series of characters 

 marking the highest development in one direction, which has been used, 

 to all appearance with great propriety, and which may be necessary to 

 complete a natural series of orders from the data now obtained. These 

 orders all arise naturally from principles which, for some time past, have 

 been admitted by the best authorities, only requiring their proper com- 

 bination to supply definitions which fulfil all the requirements for a good 

 series of orders, or primary groupes, under the class. They are : Ord. 

 I., AsiPHONiDA : Mantlelobes, and branchial lamellas unconnected, or 

 nearly so ; muscular impression generally single ; foot usually either obso- 

 lete or byssiferous. Families beginning with the lowest — 



1 Ostrseidae. 



2 Mytilidee. 



3 Aviculidse. 



4 Pectinidae. 



5 Arcade. 



Ord. II., Nayades. — Mantle margins unfled between the excurrent 

 and incurrent orifices, and rarely in front of the latter ; excurrent orifice 

 plain, incurrent fringed ; foot very large ; branchial lamellic united pos- 



