20 
which the trilobite crept out,and left his castoff 
covering in the same manner as recent crus¬ 
taceans leave their exuviae. We know that 
the limulus polyphemus creeps through a 
somewhat similar opening, made along the 
whole anterior edge of his buckler.* In all 
our fragments, which exhibit the under sur¬ 
face of the buckler, the lower lip is reflected 
beneath, so as to form a kind of scroll or roll¬ 
ed edge, extending from one side or angle of 
the head to the other. Beneath this, and 
passing backwards towards the tail, the sur¬ 
face of the shell is not flat and horizontal as 
in the isotelus and limulus ; but it swells up 
on each side, below the oculiferous prorne- 
nences, into a kind of oval pouch, diminishing 
in breadth as it recedes, and at last terminates 
in a rounded point, below the second articula¬ 
tion of the vertebral column. This is the posi¬ 
tion of the gullar pouch or plate, when the 
animal assumed a creeping or swimming atti¬ 
tude ; but when rolled up in the form of a ball, 
* See Dr. Dekay. Annals of Natural History, vol. i. 
