SECT. XIII 



THE TRILOBITES 



215 



derived from the primitive head-shield above de- 

 scribed. The bivalve shells of the Ostracocla can 

 also be deduced from the same by the clapping 

 together of the two wings of the crescent-shaped 

 ridge against the sides of the body as illustrated in 

 l'*ijv 57' P- 2 57- When this crescent is large, owing 



FIG. 48. Acidaspis Dufrenoyi (Barr), Upper Silurian (after Barrande, from Zittel's 

 Handbncli). Showing the fold of the skin carrying two prongs ^projecting 

 backwards just behind the glabella, to demonstrate the probable origin of the 

 dorsal shield of Apus. 



to the great development of the shovel-shaped ridge 

 round the front of the head, the lateral folding 

 of these wings round the rolled-up body would yield 

 a bivalve shell. Another obvious method of pro- 

 ducing the bivalve shell is by the folding down of the 

 edges of a dorsal shell such as that of Apus. These 



