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 sea coast. 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 tail^ 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- 

 cies 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 



