On Fossil Mollusca.--Numerous as are the fossil remains of the various types of animal organization which have already passed under review, they are far exceeded in number and variety by those of the beings whose mineralized relics we now propose to investigate. Although every one is familiar with the external appearance of the shells cast up by the waves on the shores of our island, and of those which, from their varied colours and elegant forms, are preserved in 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 organization of the Mollusca; but to the geologist, from their permanent nature, and the proofs they yield of the conditions 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, Eocene, Miocene, and Pliocene, (proposed by Sir C. Lyell,) have reference to this character, as we have previously explained (_ante_, p. 24). [Sidenote: FOSSIL MOLLUSCA.] The _Mollusca_, a name indicative of the soft nature of the integuments of these animals, constitute a very comprehensive subdivision of the animal kingdom, and are separable into two principal groups, viz. the _Acephala_ and the _Encephala_. I. The Acephala (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 come within the scope of our inquiries: yet it is possible that the soft parts even of these perishable structures may have left some trace, or that markings of their integument on the silt or mud may be preserved;[348] and I would recommend the student to search for such indications on the rippled surface of clays and sandstones. [348] The _Ischadites Königi_ of the Ludlow rock was supposed to resemble _Boltenia_, a pedunculated Ascidian.