8 
petrifactions may be known, and they will be found 
illustrated in a manner more or less striking, i 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. ee. 
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 ut- 
most care. A portion of the underside of a trilobite ([sotelus 
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 De Dekay, 
by some dissections of his own. , 
