8 



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 oc- 

 cur, will be distinctly marked, when the species are 

 described. 



The superior covering, or upper shell of the trilo- 

 bite is the only part of the animal, concerning which 

 we have any satisfactory knowledge. It is conjec- 

 tured that it was furnished with articulated feet, but 

 no traces of any organs of progressive motion have 

 hitherto been fairly discovered.* 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. 



That these petrifactions were once marine animals 

 there can be little doubt, for they are always found 

 associated in the same rocks with shells, and other 

 productions peculiar to the sea. 



The Trilobite is supposed by many naturalists to 



* Mr. Parkinson states, that in a trilobite which he possessed he 

 thought he perceived the points of the feet; but on endeavouring 

 to detach the piece of rock in which it was embedded, the speci- 

 men was entirely shivered, though he worked at it with the t\t- 

 most care. A portion of the underside of a trilobite (Isotelus 

 gigas) near the anterior edge of the head, was distinctly ascer- 

 tained, by Dr. Dekay, but only enough to convince him of its 

 analogy in this part with that of the limulus polyphemus no or- 

 gans of locomotion could be seen. Mr. Stokes, the distinguished 

 fossilist of London, has confirmed the observation of Dr. Dekay, 

 by some dissections of his own. 



