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7 
ticulars, and form together an exceedingly natural 
group. The body, with but few exceptions, is di- 
vided transversely into three parts. The anterior 
portion or head often resembles the buckler of the 
horse foot or king crab (limulus polyphemus), so com- 
mon on our seacoast. The middle portion is the abdo- 
men, and is always separated transversely into a num- 
ber of segments or articulations, generally diminish- 
ing in breadth as they recede from the head. The 
posterior end is the fail, which, though in some spe- 
cies, a mere prolongation of the abdomen, that can 
scarcely be distinguished from it, yet in others it as- 
sumes a genuine caudal appendage. 
The head of the trilobite is also generally divided 
into three parts: the middle is called the front, or fore- 
head; 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 spe- 
eles of insects, that there can be but little doubt that 
these tubercular projections, were true organs of vi- 
sion. 
Some of the genera which belong to this remarka- 
ble 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 
