PHORMOSOMA HISPIDUM. 91 



system as in an earlier stage (PI. 43, fig. 5), but leave a small open area in 

 the interambulacral angle between the buccal plates. An interesting fea- 

 ture of this specimen is the splitting of the primordial plate into two. This 

 splitting of the plates is carried on in older stages both in the ambulacra! 

 and interambulacral areas. That of the primordial plate occurs also in 

 older specimens (PI. 44, fig. J) in the right anterior and light posterior 

 interambulacra. In the right posterior interambulacrum (PI. 43, fig. 6) the 

 second plate is abnormal; one of the second plates is reduced to a small 

 intercalated triangular plate. 



In specimens measuring 43 mm. in diameter there are five rows of 

 buccal plates (PI. 44, fig. 2), in those of 64 mm. there are six (PI. 44, fig. •/), 

 and in one 137 mm. we find seven. With increasing size the buccal plates 

 become more elongate, the row immediately at the teeth alone retain 

 more or less their circular outline (PI. 44). In the specimens of 43 and 

 137 mm. the bare interambulacral area adjoining the primordial plate is 

 covered with a few minute, elongate, irregularly arranged plates, which 

 correspond to the interradial buccal plates of Cidaris. 



In a specimen of 53 mm. in diameter (PI. 30, figs. 9, 10) it will be seen 

 that on the actinal side there is no greater number of interambulacral plates 

 (PI. 49, fig. 3) than in a specimen not much more than half the size (PI. 49, 

 fig. J). The position of the primary tubercles in the plate indicates that 

 it has grown fairly uniformly in all directions. The scrobicular circle 

 has become well-defined in some of the plates, and they are much more 

 closely covered by small miliaries than in the previous stage. Between 

 the ambitus and the genital plates, however, there have been four addi- 

 tional plates formed ; there are thirteen in place of nine, differing in length 

 and in the greater number of small miliaries found in the plates. 



In a somewhat smaller specimen of 47 mm. the arrangement and number 

 of the actinal and abactinal plates of the corona are similar to the larger one. 



In a specimen 120 mm. in diameter (Pis. 31 ; 32, fig. 3; 49, fig. 4) there 

 are fifteen plates above the ambitus and seven below, above the primordial 

 plate. There are only eight on the actinal side of a specimen measuring 

 203 mm. in diameter (Pis. 37 ; 49, fig. 5) between the ambitus and the 

 genitals (PI. 38), showing that the increase in size takes place on the actinal 

 side mainly by the growth of existing plates in all directions, while on the 

 abactinal side the coronal plates increase somewhat in size, but greatly in 

 number also. 



