184 



PAN AM I C DEEP SEA ECHIXI. 



plates both in the ambulacral and interambulacral areas. How these are 

 formed I have been unable to observe ; they evidently greatly strengthen 

 the ambital edge of the test, Figs. 274, 275. 



The drawing out laterally of the interambulacral and ambulacral plates 

 at the ambitus of Spatangoids is characteristic of those recent genera in 

 which the actinal surface of the test is, in the interambulacral areas, occu- 

 pied by not more than three large plates. 



S'2 iiiiii. 



Fig. 274 LiNofNiiisitd longispints. 



In the Ananchytids the differences between the actinal, ambital, and 

 abactinal plates are slight ; they become somewhat marked in Cassidulids. 

 In Micraster, the lengthening of the ambulacral plates at the anal plastron 

 is very decided, as it is in all recent Spatangoids where the anal plastron is 

 well developed, except the Ananchytidoe or Pourtalesise. The lengthening 

 of the ambital plates takes a great development in such genera as Maretia, 

 Motalia, Breynia, Lovenia, and the like, where the junction of the abactinal 

 and actinal areas forms a sharp angle, somewhat as in Ilomolampas and 

 Phrissocystis, but to a less extent. In these genera the formation of a sharp 

 angle at the ambitus is due to the excessive elongation of two or three of 

 the ambital plates in all the ambulacral and interambulacral areas except 

 the anterior ambulacrum.' 



» Lovcn, fitudcs, Pis. IV, figs. 41, 42; XXXIT. 



