36 Remarks on the Trilohite. 



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 structm'e 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 aiiimal. 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 antennas 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 func- 

 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 branchiae 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 difl'ers 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 Neio 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 



