TRIARTIIRUS BECKI 199 



Another feature noticed in the fiekl is that the specimens 

 nearly all lie with the back down. The same thing has been 

 observed by other investigators, and has been accounted for 

 by the assumption that in being drifted about along the 

 bottom such a position would be assumed from the centre of 

 gravity being on the convex side. This idea does not seem 

 tenable, because, while on their backs, the trilobites would 

 be most easily rocked by the currents of water, and eventu- 

 ally be turned over or dismembered. A further explanation 

 has been offered by Hicks and accepted by Walcott,* to the 

 effect that trilobites probably lived with the ventral side 

 down, and the accumulation of gases in the viscera during 

 decomposition was sufficient to overturn the animal and allow 

 it to be buried by the deposition of sediments in the position 

 now found. This theory, also, does not meet the facts as 

 here observed, for in turning over a dead and limp animal 

 provided with long and slender antennte, delicate jointed 

 legs, and fringed appendages, the legs would be either folded 

 under the carapace on one side, or disj^laced from their 

 natural position. But, as has been already noticed, the 

 present material generally shows the legs extended on both 

 sides of the body and the antennae in a very lifelike position. 

 (Plate VI, figures 3-7.) 



It seems most probable that trilobites could both swim 

 freely and crawl along the bottom, and that, on dying, they 

 coiled themselves up in the same manner as the recent 

 isopods. Then upon unrolling they would necessarily lie 

 on their backs. Even if they did not coil up, any swimming 

 animal having a boat-shaped form would settle dowuAvard 

 through the water with the concave side up. 



The definite structure of the legs of Triarthnis is now 

 for the first time clearly shown, and is of much interest. 

 Furthermore, a difference can be seen in the appendages of 

 the pygidium, thorax, and cephalon. Those of the caudal 



* The Trilobite : new and old ovidence relating to its organization. Dull. 

 Mus. Comp. ZuiJl., VIII, No. 10, 1881. 



