and the Recent Echinid Faune. 43 
what more complicated, as the affinities of the genera from 
which we trace the development of the Echinide, the Arba- 
clade and the Salenide, is very close in the Liassic, the Juras- 
sic and the Lower Cretaceous beds; where such t pes as 
Acrosalenia, Hemicidaris, Glypticus and Rhymechinus, show us 
how readily we may pass, on the one hand, to the Salenidze 
and on the other to the Temnopleuride, the Echinide and the 
Arbaciadee. It is, however, only when the interbranching affin- 
ities have not extended in too many directions that we can still 
easily follow the systernatic connection, which is as close as we 
can possibly desire to have it. In fact it is so extended that 
Cidaride, the small number of coronal plates, the small number 
of large primary interambulacral tubercles, the variable shape 
of the primary spines, the secondary papillz, the large plates 
structural features. These last named features are all features 
which tend toward the Echinidw proper, and which thus far 
have not appeared in the older Cidaridx, though we find some 
of these characteristics already foreshadowed in the imbricating 
of the former uniform extension of the ambulacral pores as far 
as the actinosome. In the structure of the apical system, the 
subanal plates can still be traced in some of the stages of 
