PPS io dpe ee 
Spee eg EDT ae oe Sap aeRO oo eT RoR TAY PONT Ye LEE art pe eee aie tee eee en een Baas 
meee A ete aaah 
and the Recent Echinid Faune. 45 
ally forming a more or less distinct snout; and when this is 
with their simple ambulacra leads directly to the Nucleolids 
On 
q ceous types like /nfulaster, and their derivation from such forms 
_ as have been figured by Ooster (Echin. Alp. Suisses, pl. x, figs. 
_ 1-4) as Dyaster calceolatus. (See also de Loriol, Echinoid. Crét. 
de la Suisse, pl. x xxiii. 
When we take the Spatangoids of the Chalk, they lead us 
directly through the Palzeostominee and the Collyritide to the 
Ananchytide, which have persisted to the present day; and 
also to the Spatanginee proper, represented by their few genera, 
Such as Micraster, Hemiaster, and Prenaster, which already 
possess the structural features characteristic of the recent Spa- 
tangoids. That is, we find genera with a peripetalous fasciole, 
 asubanal fasciole, sunken ambulacra, petals of a different de- 
gree of development, spines specialized on certain areas of the 
_ test, a trace of a sunken anterior groove, of an anal beak, of an 
actinal plastron, of a snout, of a lateral fasciole, and of a special- 
ization of the primary and secondary tubercles. But, of course, 
_ the extent to which these features may be developed in Tertiary 
and recent genera contrasts often strikingly with the rudiment- 
ary nature of the structural features found in the Cretaceous or 
+ertiary genera. The simple actinostome of the Paleeostomine 
18 combined with a well-marked specialization of the ambulacra 
_ above the ambitus, the petaloid feature of the early Spatangoids 
which appears later in the Cassidulidse ; while in the Anane 
tide the well developed labium of all the more recent Spatan- 
goide is combined with a comparatively more rudimentary 
State of the ambulacral zones. 
Among the Cretaceous genera, Hemipneustes and Ennalaster 
are extremely instructive. They show, perhaps better than any 
_ Others, the passage which exists between the earliest Spatan- 
goids with more or less petaloid ambalacra, and the older Spa- 
tangoids without petals, and in which the ambulacra have the 
_ Same simple structure from the apical system to the actinostome. 
In both these genera the petaloid structure is limited to the 
Posterior poriferous zone of the lateral ambulacra; the only 
