CHAP. III.] THE CRUISES OF THE 'PORCUPINE.' 103 



' radioles,' as the fossil spines of Oidarites are usually- 

 called, presenting a very marked character, had been 

 found in various formations from the lower oolite 

 upwards. These spines are paddle-shaped, compressed, 

 longitudinally grooved, flattened almost into plates, 

 and strongly serrated on the edges. In the nummu- 

 litic beds of Val-Dominico near Verona such spines 

 were found associated with plates much resembling 

 those of Cidaris, but with the unique peculiarity of a 

 row of holes penetrating the test in the areolar space 

 round the primary tubercle. This character our new 

 Urchin does not possess, but the radioles have the 

 flatness, the longitudinal strige, and the serrated edges 

 of those of Forocidaris. 



I do not attach much importance to the perfora- 

 tions in the plates. From Desor's figures they are 

 not round and defined in outline, but lengthened and 

 somewhat irregular, and they radiate from the inser- 

 tion of the spine. Our species has a set of depres- 

 sions occupying the position of these perforated 

 grooves which are undoubtedly for the insertion of 

 the muscles moving the large long spines, and as the 

 test is thin these grooves might readily penetrate the 

 plate, or so nearly penetrate it as to be worn into 

 holes by very little drifting or wear. 



Our recent species and the eocene form have 

 another character in common ; the areolar circles 

 are not well defined, and the areolse tend to become 

 confluent. 



Scattered plates only of this genus have been 

 found fossil, and the ovarial plates were till now 

 unknown. They present a very singular character, 

 which is certainly of generic value. The ovarial 



