FOSSIL MOLLUSCA. 375 
_ the cottage of the peasant, and in the mansion of the rich, 
but few persons are conversant with the nature of the animals 
that secreted and were protected by these beautiful and 
enduring structures. The organization even of the oyster, 
mussel, whelk, &c., is known only to the naturalist. Appearing 
to the uninstructed eye as a shapeless gelatinous mass, there 
is nothing to arrest the attention, or excite the curiosity. 
Yet the beings which inhabited these durable cases, are 
objects of the highest interest and present a rich field of 
instructive investigation. 
Except as shedding some light on the structure and 
_ economy of their inhabitants, the shells, in the estimation of 
_ the naturalist, are the least interesting part of the organiza- 
——w 
H 
tion of the Mollusca; but to the geologist, from their 
permanent nature, and the proofs they yield of the condi- 
tions under which the strata that contain them were deposited, 
they. are important in the highest degree. It has even been 
_ found convenient to classify formations, in which fossil shells 
largely prevail, by the relative numerical proportion of the - 
- recent and extinct species found in the different groups of 
_ strata ; and the terms, Hocene, Miocene, and Pliocene, (pro- 
posed by Sir C. Lyell,) have reference to this character, as 
_ we have previously explained (ante, p. 24). 
The Mollusca, a name indicative of the soft nature of the 
integuments of these animals, constitute a very comprehen- 
sive subdivision of the animal kingdom, and are separable 
- into two principal groups, viz. the Acephala and the Lnce- 
 phatla, 
- 
I. The AcrepHaLa (so termed because they are destitute 
of a head) have neither jaws, tongue, nor a distinct mouth, 
_ They are aquatic, and are subdivided into classes, according 
_ to the modification of their integument, or of their gills, 
a. The Tunicata (from the elastic tunic, or mantle, in 
_ which they are enclosed) have no shell, and therefore do not 
