36 Remarks on the Trilobite. 
some of the viscera were placed in a cylindrical cavity running 
beneath the vertebral column, and that the side lobes were only a 
covering and protection to the soft paddles or feet placed below, 
as may be seen in a similar structure in the serolis. Each of the 
five articulations of the abdomen, the under side of which we 
have not yet discovered, was probably furnished below, on each 
side of the abdominal cavity, with organs, which performed the 
double office of feet and lungs. Now, as our fragments develope 
all the inferior surface except the portion beneath these five ar- 
ticulations of the abdomen, it is probable that our trilobite was a 
decapodous animal. Professor Brongniart long ago imagined, 
that the reason why no traces of these organs have yet been dis- 
covered, is that the trilobites held that place among crustaceous 
animals in which the antennz disappear, and the legs become 
transformed into soft paddles incapable of preservation. If this 
supposition be true, we shall in vain look for any further discove- 
ries below the upper shell of the trilobite. What affords, we 
think, increasing probability to the opinion we have just advanced 
with regard to the situation of the abdominal cavity, and the or- 
gans of locomotion below the five abdominal arches above men- 
tioned, is, that when the animal rolled itself up for protection, 
this portion of the body would still retain nearly a rectilinear 
position ; thus no interference would occur in the ordinary fune- 
tions of the animal economy when the body was contracted. 
Besides the organs of locomotion and respiration beneath the 
abdominal arches of the genus calymene, it is probable that on 
each side of the deep cavity under the caudal end there was 
placed a series of thin transverse plates, which also performed 
the combined functions of breathing and swimming: a similar 
disposition of laminated branchiw may be observed also in the 
limulus and in the serolis. Beneath this deep cavity the heart of 
the animal was also probably placed. 
What we have said with regard to the inferior mechanism of 
the trilobite, applies exclusively to the genus calymene. It is 
probable that this structure differs essentially in all the genera of 
this remarkable family. Dr. Dekay has described and figured in 
the first volume of the Annals of the Lyceum of Natural His- 
tory of New York, the under side of the buckler of the isotelus, 
which is very peculiar in its configuration ; he describes this in- 
ferior surface as being formed by the anterior part of the buckler 
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