I\rOLLUSCA PTEROrODA. lS5 



their inner surface, should he gradually removed to a great- 

 er distance from the hinge, so that it may preserve its rela- 

 tive situation with regard to the whole shell, and retain un- 

 diminished its power of acting upon the valves. For this 

 purpose its adhesions are gradually transferred, by some un- 

 known process, along the surface of the valves; and the pro- 

 gress of the removal may generally be distinctly traced by 

 the marks which are left in the shell at the places before oc- 

 cupied by the attachments of the muscular fibres. The same 

 process takes place when there are two or three muscles in- 

 stead of one. 



A few genera of Mollusca, such as the Phohts, have, in 

 addition to the two principal valves, small supplementary 

 pieces of shell. They have been accordingly comprised in 

 the order of Midtivalves. which also comprehends Cuvier's 

 order of Cirrhopoda, including the several kinds of Barna- 

 cles, (the genus Lepas of Linnaeus,) which are furnished 

 wnth a great number of jointed fdaments, or c/rr/iz, and form 

 an intermediate link of connexion between the Mollusca 

 and the Jlrticidata. But the limits of this treatise will not 

 allow me to dwell on the endless diversities of structure 

 which this subject presents. 



§ 5. Pteropoda. 



In the Mollusca belonging to the two orders which have 

 now passed under our review, namely, the Jlcephala and 

 Gasteropoda, the mantle, while it folds over the principal 

 viscera of the body, leaves apertures for the admission of 

 water to the gills, or organs of respiration. But there exist 

 a few genera having the sac formed by the mantle closed 

 on every side; a structure which renders it necessary to 

 adopt a different arrangement with regard to the gills, and 

 to place them externally, and we then find them spreading 

 out like a pair of wings, on each side of the neck. Since 

 this general closing of the mantle precludes, also, the for- 

 mation of any organ of progressive motion corresponding 

 to a foot, advantage is taken of the projection of the gills to 

 Vol. I. 24 



