MITCHELL — ON THE IIESTOIIATION OF PTERASPIS. 



405 



servation, and their exact measurements have been followed in tlie 

 figure. The first specimen, used for this diagram, exhibits very 

 beautifully the form of tlie shield with the terminal horns, and the 

 distinct eye-sockets. The eyes are placed on the margin of the shield, 

 and their impression is also seen on a cast in the stone of this spe- 

 cim.en. The second specimen 

 is the prolonged central ter- 

 mination of the shield, which 

 has been broken ofi" at the 

 ridge which terminates on 

 either side in the horns. The 

 third specimen shows the junc- 

 tion of this central prolonga- 

 tion with the shield. All the 

 three specimens have a high 

 central ridge, and still retain 

 something of the graceful out- 

 line of the living form. As 

 preserved in the stone these 

 specimens show^ only the na- 

 creous layer, the other com- 

 ponent layers of the bone of 

 the Pteraspis having perished 

 in their case. 



figure 2. Two specimens 

 have been employed for the 

 construction of this diagram, 

 the one a piece of solid bone, 

 and the other a cast in the 

 stone of the under surface of 

 the dorsal shield. They are 

 a])i)arently the relics of indi- 

 viduals of the same species and 

 even size ; and we have again 

 followed their exact measure- 

 ments in the figure. The spe- 

 cimen of solid bone exhibits 

 the external aspect and general 

 outline of the shield, and is especially valuable as showing the form 

 and position of the horns of the shield. It will be observed that 

 these are not the prolonged cusps of the Cephalaspis, or at least 

 of the Cepludaspis Lyellil. The cast shows the marks of the eye- 

 sockets, and exhibits traces of the central ridge of the shield. The 

 ridge, howx^ver, is a low one in comparison of that in Figure 1 ; 

 and w^e have hitherto failed to discover the central prolongation or 

 termination of this larger or adult form. 



Figure 3. This must be understood to be very conjectural ; aiul 

 although three of the plates are copied exactly as they occur in the 

 stone, their arran":ement and their function must be considered so far 



Fio-. 2. 



