504 PHILOSOPHY OF ZOOLOGY. 



analogy in the animal kingdom. Many species are ovovi- 

 viparous ; in which case the eggs when ripe pass into the 

 gills, where they are hatched. 



The methodical distribution of the Bivalvia appears to 

 be attended with peculiar difficulties, in consequence of the 

 uniformity which prevails in the structure and disposition 

 of their organs. The characters furnished by the shell, 

 though useful in the construction of generic as well as spe- 

 cific distinctions, have been abandoned by those who prefer 

 a knowledge of the structure, to an acquaintance with the 

 form of an animal. The characters derived from the pre- 

 sence of a byssus, a foot, or of syphons, appear to be nearly 

 of co-ordinate importance. M. CUVIER gives the preference 

 to those founded on the appearances of the latter, and dis- 

 tributes the genera into five families, an arrangement which 

 we here propose to follow. These, however, may be consi- 

 dered as occupying a much higher rank, and each as in- 

 cluding numerous families. 



1st Subdivision. 



Cloak open. There are no syphons, the anterior mar- 

 gin of the cloak being as open as the mouth of the shell. 

 When the valves open, the water comes immediately in 

 contact with the branchiae and mouth. The margin of the 

 mantle has a double fringe of filaments. 



1st Tribe. 



Valves closed by one adductor muscle. 



A. Pectenidrt. Animals free or fixed only by a byssus. 

 Furnished with a foot. 



Into this family, contemplated by LAMARCK, the follow- 

 ing ill assorted genera may be placed : Pecten, Lima, Pe- 

 dum, Plicatula, Vulsella, Placuna, Gryphsea, Perna, and 

 Crenatula. 



In the animals of the genus Pecten, represented by the 

 common scallop, there is a small foot, supported on a short 



