SECT. XIII THE TRILOBITES 223 



space in which we think the antennae should have 

 been drawn. That they were present we have httle 

 doubt, probably somewhat reduced, as in Apus, and 

 pointing backwards. Our reasons for thinking that 

 there must have been two pairs of antenna; as here de- 



FiG. 50. — Ventral surface of Calymene Senaria restored by Walcott (from ZIttel). 

 Assuming that the large pair of locomotory limbs are the sixth or first trunk 

 limbs. The two pairs of antennae are missing — they should probably be drawn 

 in on each side of the prostomium projecting backwards, as in Apus. 



scribed, are two, apart, that is, from the general reasons 

 founded upon our theory of their relationship to Apus 

 through common descent from a bent Annelid. 



(i.) Figs. 51 and 52 are sections passing through 

 the prostomium (labrum or hypostoma) of two Trilo- 



