. 
884 Agassiz—Paleontological and Embryological Development. 
opment of the bourrelets and phyllodes een a simple circular 
actinostome, the gradual deepening of slight anterior 
groove of some early Spatangoid to nae the deeply sunken 
actinal groove. Hqually well we can trace the modifications of 
the ambulacral system as it passes from the simple poriferous 
zones of the earlier Spatangoids to genera in which the petal- 
iferous portion makes its appearance, and finally becomes the 
specialized structure of our recent Spatangoid genera, such as 
Schizaster, Moira, and the like. Fin ally, we can trace to a cer- 
prominent, to genera like Brissopsis, Brissus, and the like, of 
the present day; on the other, perhaps, or in both combined, 
the formation of a lateral and anal fasciole from genera like 
Micraster in Spatangus and Agassizia. us we must, on the 
same theory of the independent modifications of special struc- 
tural features, trace the many and complicated affinities which 
so constantly strike us in making comparative studies, and 
which render it impossible for us to express the manifold 
affinities we notice, without taking up separately each special 
structure. Any attempt to take up a com ination of charac- 
ters, or a system of Perma is sure to lead us to indefinite 
problems far beyond our power to grasp. 
In the oldest fossil Giypeaiteoiie oa Petalosticha, as well as 
in the Desmosticha, we also find the potential expression of 
the greater number of ‘the modifications subsequently carried 
out in genera of later date. The semipetaloid structure of 
some of the earlier genera of a, the slight modifica- 
tions o — of the plates of the actinal side near the actinos- 
tome, the precursors, the one of the highly complicated 
elaioa aaese of the recent Spatangoids, the other of the 
actinal plastron, leading as it does also to the ot ee differ- 
ences subsequently developed in the anterior and posterior ex- 
tremities of the test, as well as to the modifications which lead 
to the existence of a highly labiate actinostome. The appear- 
ance of a few srisasiant = the actinostome constitutes the 
first rudimentary bourre 
Going back now to ite Palechinidee, the earliest representa- 
tives of the Echini in paleeozoic times, ‘without ee a to 
trace the descent of any special type from them er- 
haps find some clew to the probable Madificelsogs ok ag 
principal structural features preparatory to their gradual dis- 
appearance. In the structure of the coronal plates, the special- 
ization of the actinal and abactinal systems, the conditions of 
the ambulacral system, we must compare them to stages in the 
embryonic development of our recent Hchini with which we 
find no analogues in the fossil Echini of the Lias and the sub- 
