In some varieties of rocks there is often found the fossil remains of an animal which bears some resemblance to certain species of the crab. The back of this organic relic is commonly divided by two deep grooves or furrows, into three longitudinal lobes, and from this circumstance, the term _Trilobite_ has been applied as a family name to distinguish this whole race of beings. This general appellation, however, though in most of the species, highly appropriate, is by no means applicable to all. The individuals which compose the family of the trilobites resemble each other in many important particulars, and form together an exceedingly natural group. The body, with but few exceptions, is divided transversely into three parts. The anterior portion or head often resembles the buckler of the _horse foot_ or _king crab_ (_limulus polyphemus_), so common on our sea coast. The middle portion is the _abdomen_, and is always separated transversely into a number of segments or articulations, generally diminishing in breadth as they recede from the head. The posterior end is the _tail_, which, though in some species, a mere prolongation of the abdomen, that can scarcely be distinguished from it, yet in others it assumes a genuine caudal appendage. The head of the trilobite is also generally divided into three parts: the middle is called the _front_, or forehead; and the lateral portions the _cheeks_. In most cases, a projecting tubercle, or knob, is observable on the anterior surface of each cheek, which has much the appearance of an eye. Its reticulated structure is in many instances so analogous to that of the eyes of some crustaceous animals, and also of some species of insects, that there can be but little doubt that these tubercular projections, were true organs of vision. Some of the genera which belong to this remarkable race of fossil animals, possessed the power of rolling or coiling themselves up into a kind of ball, like certain species of insects, or like the armadillo; and they are always found embedded in the rocks in this attitude. Such are the general characters by which these petrifactions may be known, and they will be found illustrated in a manner more or less striking, in most of the species. The exceptions, which rarely occur, will be distinctly marked, when the species are described. The superior covering, or upper shell of the trilobite is the only part of the animal, concerning which we have any satisfactory knowledge. It is conjectured that it was furnished with articulated feet, but no traces of any organs of progressive motion have hitherto been fairly discovered.[2] Hence, it may be reasonably supposed, that the structure of the lower portions of the animal were so soft and delicate, as to render them incapable of sustaining the process of mineralization, which the hard crustaceous covering of the back so successfully undergoes.