174 SYNOPTIC COLLECTION. 



condition of the two rowed ambulacra and the one rowed 

 interambulacra. Later some of the ventral plates are 

 resorbed causing more or less irregularity in the shape 

 of those that are left, while the t\\o rowed interambulacra 

 arise and remain essentially unchanged. 



The ambulacra of the adult are narrow with a single 

 nearly vertical row of paired pores. The interambulacra 

 on the other hand are broad and carry the primary spines 

 which are large and few in number. 



The anus is somewhat raised above the anal disc. 

 Surrounding the latter is the ring of genital and ocular 

 plates, the genitals pointing outward and the triangular 

 ocular plates inward. 



The spines of this species are cylindrical. Some are 

 young and short with distinct vertical ridges on the sur- 

 face, while the older ones are long and are entirely cov- 

 ered with a growth of algae, etc. Very different from 

 these are the modified spines which are found on the 

 abactinal surface of the corona and which also crowd the 

 actinal area. These are like short, stout, flattened clubs. 



It has been seen that the Cidaridae of the present era 

 retain in their youth many of the primitive characters of 

 the ancestral Bothriocidaris. The changes that convert 

 the young into the adult are an increase in the number of 

 coronal plates, the differentiation of the actinal and abac- 

 tinal areas from the rest of the corona. and the modifica- 

 tion of the spines. 



While the Cidaridae represent one division of ancient 

 Echinoids, another and more specialized division includes 

 the Melonitidae. The generalized members of this family 

 are Rhoechinus and Palaeechinus, and the specialized are 

 Oligoporus and Melonites. 



Although there is a slight overlapping of the ambu- 

 lacral plates in Rhoechinus, as seen in PI. 318, fig. i, owing 

 to the fact that these plates are not united along their 

 edges, still they may be said to extend across one half of 

 the ambulacral area as in the ancestral form so that only 

 two rows of ambulacral plates exist. 



