226 M.A. Pomel on the Classification of Echinida. 
and the obovate form mask the radial symmetry for the be- 
nefit of the bilateral. The second have the mouth central or 
nearly so, the ambulacra similar, and the anus more or less 
posterior, but often mounting high enough to enter into the 
series of the genital pieces, which open behind to receive it. 
The third have the mouth central and the anus opposite, 
always completely enclosed by the genital apparatus. This 
division appears to us more natural than the division into 
two groups of regular and irregular forms, the distinctive 
character of which is not so absolute as it has been repre- 
sented. 
The Spatiformes or Spatangoides form two groups, according 
as their ambulacra are constituted by simple pores throughout 
their whole extent (Ananchytida), or as these ambulacra are 
petaloid (Spatangi). 'The*homogeneity of the family of true 
Spatang? is such that no one has yet supposed that it could 
be subdivided; however, an attentive study permits us to 
recognize and even to define in it several new well-marked 
groups. 
1. The Hupatagia have the madreporic tubercle prolonged 
behind between the ocellar pieces in the place of the unpaired 
genital plate ; their petals are even with the test, and provided 
on the interporiferous area with tubercles like those of the 
other areas; we may further separate in it the type with an 
internal fasciole (Breynia), that with lanceolate petals (Hupa- 
tagus), and that with sublinear petals (Zrachyspatagus). 
2. The Brissia are like the foregoing, as far as the ma- 
dreporide goes; but their petals are depressed, well-defined, 
and with an interporiferous zone provided only with granules. 
Some have the tubercles of the back heterogeneous (Lesk7a) ; 
others have them almost homogeneous, and their periprocta is 
remarkably open (Brissus) ; others have some small, oblique, 
closely approximated dorsal tubercles and a moderate peri- 
procta (Brissopsis). 
3. The Micrasteria have the apical apparatus compact— 
that is to say, with the madreporic tubercle in the centre of 
the genital plates, which are contiguous. The tubercles of the 
back are most frequently scattered amidst an abundant granu- 
lation. The ambulacra are depressed and well defined. Al- 
most all the genera have fascioles. 
4, The Yoxasteria have the apex compact, and some 
scattered dorsal tubercles; but their petals are even with the 
test, and the anterior ambulacrum has linear pores, either 
alone or mingled with round pores. The peristome is not so 
boldly labiated as in the other Spatangoides. There are no 
fascioles, 
