698 THE PALEONTOLOGY OF MINNESOTA. 
(Terminology. 
The pygidium, P, consists of a number of anchylosed segments, their parts on 
the axis being known as annulations, an, and on the pleure as ribs, pr. The ribs may 
terminate within the margin of the shield or be produced beyond it. The extremity 
of the pygidium may be obtuse, angular or continued into a caudal spine, es. 
The test of all the parts is continued beyond and beneath the margin, the enfolded 
portion or doublure extending but a short distance inward. This is trans-sected by 
the anterior limbs of the facial suture, or by a branch or branches from those limbs. 
That portion of the doublure lying beneath the anterior extremity of the head and 
sometimes isolated by the sutures is the epistoma. Below this lies a free subquadrate 
or subtriangular plate, the hypostoma. The doublure is more or less distinctly 
developed at the extremities of the thoracic segments and within the margin of the 
pygidium. 
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