158 AFFINITIES AND GEOGRAPHICAL 



pears that the Brachiopoda, which are the predominant fossils 

 of the Lower Silurian era, are the first animals we meet with 

 in this line, having parts capable of commemorating their ex- 

 istence. While the Brachiopoda are generally inhabitants of 

 deep seas, the Lamellibranchiata, among which are included 

 the oyster, muscle, and other testacea, affect the beds of shal- 

 low seas, whence they spread in a variety of genera, towards 

 shores, the mouths of rivers, and into fresh water. The 

 Lamellibranchiates are higher than the preceding class ; they 

 are the first bivalves which possess a true hinge. It is also 

 remarkable that, with the decline of the brachiopods, at an 

 early point in the secondary formation, rises the lamellibran- 

 chiate class. There is here, therefore, an improvement in 

 organization, an advance in habitat landward, and a succession 

 of existence in the geological ages, all in harmonious connec- 

 tion. NOT is this all. The lamellibranchiata are again di- 

 visible into monomyaria arid dimyaria, the former having one 

 adductor muscle, and the latter two ; the former, moreover, 

 being intermediate between the brachiopods and dimyaria in 

 respect of non-symmetrical form. Now the monomyaria 

 succeed the brachiopods as an abundant and predominating 

 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 

 organization 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 Pteropoda, 

 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 

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

 abundance in the circumpolar ocean as to form the chief food 



