4 EOCENE MOLLUSCA. 
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 Hrycinella ovalis is less than aline in its greatest diameter, while a 
specimen of Zridacna 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 Kellia pumila is a fraction of a grain, while 7ridacna 
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. 
sivalves 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 are no longer in existence, exceed in 
number those of the present day, yet at no single period will they bear a comparison witli 
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 being more highly 
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 Bowerbauk, the shell 
is formed by the secreting action of the epithelial cells covering the mantle of the 
animal, andit 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 Paleontologist, 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 the Genus Pecfen, as also some in tle Genera Arca and Peetunculus, have the margin of 
the mantle studded with spots, ‘‘oce/li,’ and these are said to be rudimentary eyes, but they are very 
imperfect organs of vision. 
i; 
