302 PROCEEDINGS OF THE ACADEMY OF [1897. 



chiton is not a primitive Chiton, but represents the last of a series 

 which has undergone successive modification, by which the tegmen- 

 tum, orginally the same size as the articulamentum, lias gradually 

 disappeared. Paleontological evidence also supports such a con- 

 clusion. 



EXPLANATION OF PLATE VIII. 



Fig. 1. Anterior portion of the mantle showing the tegmentum of 



the first and second valves. This shows the accurate 



arrangement of the spines in this region. X 12. 

 Fig. 2. First and second valves showing the size of the tegmentum. 



X 6. 

 Fig. 3. Young Cryptochiton showing the tegmenta and sutural tufts. 



X 3. 

 Fig. 4. A group of spines seen from above, with the characteristic 



arrangement of the small mantle spines (represented by 



dots) about the papillae. 

 Fig. 5. A cross section through a tuft (as in fig. 4) showing the 



development of the spines. 



