THE CARAPACE. 15 



any neural plate. This vertebra also has not 

 a persistent neuro-central suture, the neural arch 

 being firmly grown into continuity with the 

 centrum. Its remarkable prezygapophyses, 

 which articulate with the postzygapophyscs {pos- 

 terior articular processes^ of the last cervical ver- 

 tebra, have already been mentioned (ii, a). 



b. The centrum of the second dorsal vertebra is 

 covered on the dorsal side, for its anterior two- 

 thirds, by its own neural arch, the two uniting 

 by a persistent neuro-central suture ; the pos- 

 terior third of the centrum is overlapped by, 

 and articulates with, the neural arch of the 

 vertebra behind. Its neural spine expands to 

 form the first neural plate of the carapace. 



c. The arrangement of the parts of the dorsal 

 vertebrae from the third to the eighth, inclu- 

 sive, is pretty much like that found in the 

 second ; but in each the neural plate reaches 

 as far back as the posterior margin of the cen- 

 trum, although the laiiiince. of the neural arch 

 of the ninth vertebra overlap the posterior 

 part of the centrum of the eighth. 



d. The neural plate of the ninth dorsal vertebra 

 reaches back and overlaps the tenth vertebra, 

 which has no neural plate of its own. 



18. Examine carefully a Costal Plate — the third, for 

 example. Note the dentated sutures on each of its 

 margins ; the continuity of the neck of a rib with its 

 ventral surface ; the process which extends from its 

 outer end to be inserted into a pit in the corresponding 

 marginal plate. In partly grown specimens this process 



