704 



ME. T. T. GEOOM ON SOME 



The plates are widest at the equator, but regularly increase in 

 height from the peristome up to the highest point seen. 



Thus on the under surface we have elongated plates with several 

 tubercles, while on the upper surface the plates are deep, bearing one 



Pig. 1. — Portion of an Interambulacral Area of Pel an echinus 



corallinus. x 2. 



a. Plates on oral surface. m. Median interambulacral line. 



b. Plates on aboral surface. p. Peristomial notch. 



primary tubercle, the greater part being covered with minute tubercles 

 and granulations. 



The plates end abruptly at the peristome in a rounded prominence 

 separated from the ambulacra by moderately deep peristomial notches. 



The plates on the aboral side of the test seem to have been free 

 and movable upon one another, as they have commonly separated 

 along the lines of suture, and the sutures are seen at the equator to 

 be the expressions of clean vertical cuts whose edges are often some- 

 what smoothed off, while the uppermost plates have their borders 

 well rounded. This, I believe, explains the fact that the apical 

 portion of the test is wanting in all specimens hitherto found, though 

 the rest is so well preserved. The shape and disposition of the 

 interambulacral plates undergo a remarkable change on the upper 



