68 RHmOBRISSUS MICRASTEROIDES. 
Spatangoids in the geological series. The thin, narrow, ill-defined peripeta- 
lous fasciole crosses the petals without affecting their structure, as it does in 
all tlie recent Spatangoids. The ambulacra are not petaloid, the ambulacral 
plates are large, the pairs of pores are distant, and extend nearly to the 
ambitus. Homolampas fulvu A. Ag., among the Challenger Echini, has 
similar embryonic lateral ambulacra, with large plates, and flush with the 
test, Gualteria alone among the fossils representing a similar stage of this 
structure in Rhinobrissus. 
The indefinite peripetalous fasciole existing with a well defined broad 
and prominent subanal fasciole indicates that in Spatangoids the fascioles 
either may have become developed from the peripetalous fasciole, first 
making its appearance in such genera as Hemiaster, in which, however, the 
subanal, lateral, and anal fascioles are wanting ; or may have developed 
mainly from the subanal fiisciole in such genera as Micraster ; Rhinobris- 
sus in the stage here figured (PL XXIII. Figs. 3, 4) I'epresenting a Micraster 
stage to which has been added an indistinct peripetalous fasciole, while 
Periaster would represent the Hemiaster stage with rudimentary subanal 
and anal fascioles. Another species of the genus collected by the Challenger 
represents a later stage of development, with sunken lateral ambulacra, a 
peripetalous, an anal, and a subanal fasciole indicating affinities with the 
more specialized recent. Schizasteridce. 
In a fragment of the upper part of the test, which undoubtedly belonged 
to a specimen of this species measuring at least 30 mm. in length, the lat- 
eral ambulacra were as yet scarcely sunken, and almost flush with the test, 
the anterior ambulacrum, however, towards the ambitus, being indented 
and sunken, much as in the genus Homolampas. This fragment is interest- 
ing, as it shows that the peripetalous fasciole is not continuous, disappearing 
in the anterior interambulacral areas before it reaches the odd ambulacrum. 
The subanal fasciole, judging from a fragment of that part of the test, must 
have been remarkably prominent. 
It is possible that a young Spat.angoid which I figured in the Revision of 
the Echini (PI. XIV. Fig. 11) may turn out to be the young of this species 
of Rhinobrissus. It has, like this species, a peripetalous fasciole crossing the 
petals without modifying their structure ; and large ambulacral plates. It 
differs from it, however, in having a continuous lateral fasciole passing under 
the anal system, as in Agassizia. 
This species of Rhinobrissus {B. micrastcroides) will probably form the basis 
