In Tin American Geologist. January, 1894 
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 bot- 
tom, such a position would he assumed from the center <>r 
gravity being on the convex side. This idea does not seem 
tenable, because, while on their hacks, the trilobites would be 
most easily rocked by the currents of water, and eventually 
he turned over or dismembered. A further explanation has 
been offered by Micks and accepted by Walcott,* to the effed 
that trilobites probably lived with the ventral side down, 
and the accumulation of gases in the viscera during decompo- 
sition 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 ob- 
served, for in turning over a dead and limp animal provided 
with long and slender antennae, delicate jointed legs, and 
fringed appendages, the legs would be either folded under the 
carapace on one side, or displaced from their natural position. 
But, as has been already noticed, the present material gener- 
ally shows the legs extended on both sides of the body and 
the antenna- in a very life-like position. (Plate in, 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 iso- 
pods. 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 downward 
through the water with the concave side up. 
The definite structure of the legs of Triarthrus is now for 
the first time clearly shown, and is of much interest. Further- 
more a difference can be seen in the appendages of the pygid- 
ium, thorax, and cephalon. Those of the caudal region 
overlap each other, and are furnished with very long hairs, 
or seta 1 . The appendages of the head include the antenna- 
and the mouth parts, the latter consisting of the mandibles 
and maxilla' bearing palps and setae. 
The legs of the thorax have been worked out in detail, and 
*The Trilobite: new and old evidence relating to its organization. 
Bulletin of the Museum of Comparative Zoology, vol. viii, No. 10, 1881. 
