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 



P' 



FIG. 1 Diagrammatic figure of a trilobite. 



V. THORAX. 



sg. segment. 



ag. articulating groove. 



pg. pleural groove. 



P. PYQIDIUM. 

 an. annulation. 

 pr. pleural rib. 

 cs. caudal spine 



C. CEPHALON. 

 ;/. glabella 



/. frontal lobe. 



1, 2, 3. 1st, 2d and 3d lateral lobes. 



l',2',3>. 1st, 2d and 3d lateral furrows. 



x. fixed cheeks. 



fc. free cheeks. axis - 



. facial suture. P pleunr. 



e. eye. 



n. 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. Each 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 axis, a. The lateral longitudinal lobes 



