SECT. XV CLASSIFICATION OF CRUSTACEA 



257 



continue to feed and breathe, and yet remain rolled up 

 and sufficiently protected against its enemies. It is clear 

 that this end would hardly be attained in those cases 

 in which a large solid pygidium closed against the 

 head-shield. But on the other hand, it would be 

 quite possible by the longitudinal folding of the lateral 

 wings of the head-shield, as shown in Fig. ^J B. 

 We may well suppose that some Trilobites adopted 

 this method of protecting themselves, since, besides 



Fig. 57. — Harpes ungula Sternb. A, dorsal view; .Scrolled up in profile (from 

 Bronn's Klasscn mtd Ordiiiuigen des TJiicrreichs) ; B, to show the probable 

 origin of the Ostracoda, the head-shield with the enormously developed frontal 

 fold, shown here in profile, only requires to bend in the dorsal middle line to form 

 a bivalve shell. 



the great advantages already mentioned of allowing 

 the animal still to use its limbs and to move about 

 and feed while remaining almost perfectly enclosed, 

 it is also clear that the closing of such bivalve 

 shells, which would never be very wide open, would 

 be a much quicker and simpler process than the 

 rolling up of the whole body in the sagittal plane. 

 In assuming this origin for the bivalve shell of the 

 Ostracoda, and not • that from the folding down of a 



