SECT, xin THE TRILOBITES 215 



derived from the primitive head-shield above de- 

 scribed. The bivalve shells of the Ostracoda 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 

 ig- 57> P- 2 57- When this crescent is large, owing 



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

 Handbnch). 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 



