PLATE XXVIII. 



surface of the dark blue space lying within the shell, immediately 

 below the hinge, and extending from thence about one fourth part 

 of its whole length. This is the region in which the animal is 

 attached by its ligament to the valves of the shell ; besides which, a 

 gloss of pearly hue is observed to pervade the whole of the inner 

 surface, only that it is most conspicuous in the darker disk of the 

 shell, Asa secondary character this pearliness is very remarkable in 

 the shell before us, at the same time that as a generical denomination 

 the term Margaritifera assigned to it from this circumstance alone is 

 liable to objection ; because, the same pearliness prevails in many 

 shells which have no relation whatever with the present, either in the 

 form or structure of the hinge, and it is to these we must resort for 

 its true essential character. 



Lamarck constitutes many genera of the shells included in the 

 Ostrea genus of Linnaeus. His Mall^ac^es comprehend five genera, 

 Crenatula, Perna, Malleus, Avicula, and Meleagrina, all which are 

 allied more or less remotely to the shell before us. To that particular 

 family which is known among collectors by the designation of Hammer 

 Oysters, he gives the name of Malleus, in the French Marteau, both 

 alike implying the hammer like form of the species Malleus, which 

 Lamarck assumes as the type of this genus. But even there after 

 all the renovation that has been attempted, the result is not 

 satisfactory, because this figure is by no means constant, even in the 

 few species included by its author in that genus ; it contains but six 

 species, and these are entirely at variance with each other. Thus for 

 example, in Malleus Vulgaris, the common Hammer Shell, we have 

 a species with three lobes, a lateral one of considerable size being 

 advanced on each side the beaks: and another shell of the same 

 species with only short lateral lobes instead of large ones. Admitting 



