DISTRIBUTION OF ORGANISMS. 



179 



non-symmetrical form. Now, in the Geological Chronicle, the 

 monomyaria succeed the brachiopods as an abundant and pre- 

 dominating form, and are succeeded again, in that respect, by the 

 dimyaria. This beautiful harmony between the fossil history 

 of the acephalous mollusks and their order in progressive or- 

 ganization, is expressly declared by M. Agassiz. 



The three highest molluscan classes, univalved, possessing 

 heads, and with hardly an exception destined for independent 

 locomotion, stand apart from the bivalve orders ; generally 

 superior in organization, as beseems their higher destiny, but 

 not on that account to be held as an advanced form in the 

 same genealogy. The lowest univalve class called the Ptero- 

 poda, from their mode of progression by a couple of wing-like 

 membranes projecting from the neck may be described as 

 marine slugs, generally of small size, many of them naked, 

 others protected by a very delicate shell, which swim through 



FIG. 94. 



Jc 



Existing forms of Pteropods. 

 A, Hyalcea; B, Criseis; C, Clio. 



the ocean in vast multitudes ; one species (clio) being in such 

 abundance in the circumpolar ocean as to form the chief food 



