216 
But Bothriocidaris is by far the most important in this connection 
and seems to me definitely to dispose of Jackson's interpretation 
of the interambulacra. The genital plates are thus not thereby 
deprived of any part of their morphological importance, and the 
other arguments adduced by Jackson for regarding these plates 
as being only of secondary morphological value - do not appear to 
me to give better support for this opinion. These arguments are: 
that the madreporic pores may sometimes extend beyond genital 2, 
to which plate they are typically limited, and that the genital 
pores are sometimes placed outside the genital plates. It must be 
emphasized that the primary madreporic pore is, so far as known, 
always connected with genital 2. How the fact that the pores may 
sometimes in later stages extend also to some neighbouring plates, 
while still occupying the genital 2, can alter anything in the 
primary relation between the madreporite and the genital 2, I am, 
indeed, unable to see. Likewise I do not see that the fact of the 
genital pores being sometimes placed outside the genital plates, as 
in the Clypeastroid Peronella peronii, can be of so eminent im- 
portance. But the relation of the genital pores and the genital 
plates is, upon the whole, scarcely of so primary importance. Witb- 
out entering upon the question of the general homology of the 
genital plates of the Echini, I would decidedly maintain, against 
Jackson, that these plates are really of primary importance. for 
the morphology of the Echinoids. 
The apical system of Echinoids has been made the object of 
very extensive studies, and- the report on these researches makes 
one of the most extraordinary chapters in Jackson's work. More 
than 50,000 specimens of recent and Mesozoic Echini have been 
carefully examined and tabulated as regards the arrangement of 
the apical plates, and the results of the study of all the variants 
are very remarkable. It is shown that the generally expressed 
view that more oculårs are insert in large specimens than in the 
Young is without foundation. "All the evidence goes to show that 
the full number of oculars that are to become insert are developed 
