696 THE PALEONTOLOGY OF MINNESOTA. 
(Terminology. 
TERMINOLOGY. 
As a peculiar terminology of the parts of the trilobite has been generally 
adopted it will be useful to the student to recount in brief the signification of the 
terms employed in the following descriptions. 
The trilobite derives its name from the longitudinal lobation of the test into 
three parallel divisions. The central division alone covers the vital and essential 
organs of the animal, the lateral portions being virtually protective integumentary 
expansions. Transversely the test is also composed of three parts, a head-plate, or 
Fie. 1—Diagrammatic figure of a trilobite. 
C. CEPHALON. T. THORAX. P. Py@iprum. 
g. glabella sg. Segment. an. annulation. 
f. frontal lobe. ag. articulating groove. pr. pleural rib. 
1, 2,8. 1st, 2d and 3d lateral lobes. pg. pleural groove. es. caudal spine 
1!,2!,3!. 1st, 2d and 3d lateral furrows. 
«x. fixed cheeks, 
fe. free cheeks. oak a. aXis. 
s. facial suture. p. pleure. 
é. eye. 
nm. palpebral lobe. 
og. occipital groove. 
or. occipital ring. 
b. border. 
cephalon, C, which corresponds in a certain sense with the cephalo-thorax of the crab 
and lobster; a median segmented portion or thorax, T, and a tail-plate, or pygidium, 
P. Kach of these parts is in articulation with that adjoining. The central longi- 
tudinal lobe of the body is called, in its extent over the cephalon, the glabella, g; on 
the thorax and pygidium, it is known as the azis, a. The lateral longitudinal lobes 
sn 
