4 AGNATHA. 



dorsal plate at each side in an open suture. The dorsal armour 

 is often fused into a continuous cephalothoracic shield ; and 

 the tail seems to have always remained flexible, either with or 

 without a strong squamation. 



The mouth must have been situated on the ventral aspect, 

 with the nasal sac (or sacs) immediately in front ; and in most 

 cases it is evident that the gill-pouches opened outwardly into 

 a covered space communicating with the exterior by a single 

 hinder orifice. The position of the eyes is also always distinct, 

 but the arrangement of the other soft parts can only be pro- 

 visionally determined by inference. The impressions on the 



FIG. 2. 



Cyathaspis sturi; natural mould from visceral face of dorsal shield, nat. size. 

 U. Silurian ; Galicia. au., supposed impressions of auditory organs ; 

 br. , impressions of branchial sacs ; pin. , pit for pineal body. The eyes 

 would be situated laterally near the extremities of the bent transverse line. 



visceral aspect of the dorsal shield of Cyathaspis (fig. 2), how- 

 ever, are very suggestive in this connection. A small deep 

 pit (pin.) between the eyes occupies the precise position that 

 would have been held by the pineal body of a vertebrate brain, 

 had such been present. Further back there is a pair of 

 > -shaped impressions, such as might result from contact with 

 ridges upon the auditory capsules, due to a great development 

 of the semicircular canals, as in sharks. On either side, 

 extending nearly as far forwards as the position of the orbits, 



