4 EOCENE MOLLUSO/V. 



of abode, many remain all their lives in a stationary condition ; some are moored by a 

 cable or byssus, secreted for that purpose, by the foot ; others are located in an excavation 

 which is formed when young, and gradually enlarged to supply the increasing wants of 

 the growing animal. In regard to magnitude, they present a large amount of varia- 

 tion ; the adult shell of Erycinella ovalis is less than aline in its greatest diameter, while a 

 specimen of Tridacna gigas in the museum at the India House has a longitudinal diameter 

 of four feet, giving a difference of more than five hundred in linear dimensions ; and the 

 weight of a united pair of valves of Kelliapumila is a fraction of a grain, while Tridacna 

 is said, with its animal, to exceed six hundred pounds, and these extremes, though 

 not quite equal to those of the Vertebrata, do not fall very far short of them. 



Bivalves in the early period of the world's existence constituted a much larger pro- 

 portion of the Mollusca than the Univalves (if that designation be confined to the Gas- 

 teropoda), although much inferior in number to the Brachiopods. In these proportions, 

 however, very considerable alterations take place as we approach the present time, in 

 which the Bivalves are in the minority compared with the Univalves, but largely in excess 

 of the Brachiopods, the Univalves appearing to have taken the place of the Tetrabranchiate 

 Cephalopods, which were extensively developed during the earlier and middle periods, but 

 have now, with the exception of one genus, disappeared. The Brachiopods materially 

 diminish while the Bivalves as steadily increase up to the Tertiary periods ; and although col- 

 lectively the Bivalve species that have lived, but arc no longer in existence, exceed in 

 number those of the present day, yet at no single period will they bear a comparison with 

 existing species until we approach the older Tertiaries, their proportions during that 

 period being not very different to what they are at the present day, if we take into con- 

 sideration the comparatively limited areas that have been examined in search of fossils. 



Bivalves succeed the Univalves in a natural arrangement, the latter beins more hisrblv 

 organized, having an imperfect head with eyes more or less developed ;* the former 

 have, however, a mouth and digestive apparatus deeply inclosed within the mantle 

 and its calcareous covering. According to Drs. Carpenter and Bowerbank, the shell 

 is formed by the secreting action of the epithelial cells covering the mantle of the 

 animal, and it is enlarged solely by the increase to the margin of the shell, that is, by a 

 rib or band of shelly matter being added to the external edge of the previously formed 

 shell. The mantle or cloak which envelops the viscera, though not the most vital organ, is 

 the most important one to the Palaeontologist, as by this the shell is formed, and on this 

 the shell is moulded, and the species is determined by what this mantle has deposited. 

 This calcareous covering is exceedingly variable in its composition and solidity; in some 

 species the animal appears to possess the power of secreting a large amount of mineral 

 matter, in others the shell is particularly thin and semi-transparent ; in some it is of 



1 Species in (he Genus Pecten, as also some in tlie Genera Area and Pecftincuhis, have the margin of 

 the mantle studded with spots, "ocelli" and these are said to he rudimentary eyes, hut they are Yery 

 imperfect organs of vision. 



