CIDABIDjE. 9 



ambulacra! plate ; this is well seen in interior views of the ocular and 

 the adjoining abactinal ambulacral plates (Pis. 8, figs. 1-4 ; 12, fig. 8). 

 The young abactinal interambulacral plates, at first wedged in between 

 the oculars and genitals, are more or less connected with them by elongated 

 calcareous threads (PL 4, fig. 5) covered by compact shining nodules. 



In Dorocidaris panamemis, where the number of coronal plates is smaller, 

 the addition of new interambulacral plates takes place more slowly than in 



25 mm. 



Fig. 28. Doeocidakis panamensis. 



Porotidaris. In a specimen of 10 mm., Fig. 26, with six and seven rows 

 of buccal plates, the primordial plates have disappeared, and the first set 

 of interambulacral plates are adjacent to the actinal system in all the in- 

 terambulacra (PI. 3, fig. 1). There are thirty to thirty-two pairs of ambu- 

 lacral plates, and five pairs of interambulacral plates, the sixth being more 

 or less developed in all the ambulacra, the least in the left posterior 

 interambulacrum. In a specimen of 18 mm. (PI. 3, fig. ..') with ten rows 

 of buccal plates and forty-five pairs of ambulacral plates, Fig. 27, the 



