172 SYNOPTIC COLLECTION. 



life of Bothriocidaris. It is a primitive and an extremely 

 important stage, illuminating the otherwise obscurely 

 complex structure of the specialized Echinoidea. For 

 this reason it is called by Dr. Robert T. Jackson the 

 protechinus stage. 1 



As we have already said, the anus is opposite the 

 mouth. It is surrounded by plates, outside of which are 

 ten terminal plates of the ambulacral and interambulacral 

 areas. Each ambulacrum and interambulacrum ends in 

 one plate but none of these plates have pores. Thus it 

 is seen that there is little differentiation of the abactinal 

 area from the corona proper. 



Summing up the distinguishing characters of this 

 ancient Echinoid we have the following: A small 

 number of plates in the globular corona; slight differen- 

 tiation of the actinal and abactinal areas from the corona 

 proper ; a small number of simple spines. 



If now we come to the present time and examine Goni- 

 ocidaris canaliculata A. Ag., we find in the young (PL 

 315, fig. i; fig. 2, side view of same) some instructive 

 structural features. 



Around the mouth is a circle of ambulacral plates, 

 while the circle next to this one has ten ambulacral plates 

 (fig. i) and five interambulacral plates (fig. i, i') as in 

 Bothriocidaris. Therefore it is true that each ambulacrum 

 in Goniocidaris arises from two plates, and each interam- 

 bulacrum from one as in the ancient genus. 



The individual plates of the ambulacra are hexagonal 

 and nearly on a level with the hexagonal interambulacral 

 plates as in Bothriocidaris, but unlike this genus each plate 

 has only one pore. 



The similarity in structure between the young Goniocid- 

 aris and the adult Bothriocidaris is striking and of value 



1 We are indebted to Dr. Jackson for many of our figures and 

 facts concerning fossil Echinoidea. See Bull. Geol. Soc. Amer., 

 VII, 1896, pp. 135-170; also pp. 171-254. 



