SECT. XIII 



THE TRILOBITES 



215 



derived from the primitive head-shield above de- 

 scribed. The bivah^c 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 

 Fig. 57, p. 257. When this crescent Is large, owing 



v,n .R_AridasDlsDufrenoyI(Barr), Upper Silurian (after Barrande, from Zittel's 

 AwW^f ^ Showin" the fold of the skin carrying two prongs projecting 

 SXwardf just behind the glabella, to demonstrate the probable ongm 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 

 edo-es of a dorsal shell such as that of Apus. These 



