SCIENCE. 
143 
young Echini has been traced in a sufficient number of 
lamilies to enable me to draw the parallelism between these 
various stages of growth and the paleontological stages in 
a very different manner from what is possible in other 
groups of the animal kingdom, where we are overwhelmed 
with the number of species, as in the Insects or Mollusks, 
or where the paleontological or the embryological terms of 
comparison are wanting or very imperfect. 
Beginning with the paleontological history of the regular 
Sea-urchins of the time of the Trias, when they constituted 
an unimportant group as compared with the Crenoids, we 
find the Echini of that time limited to representatives of 
two families. One of these, the genus Cidaris, has con- 
tinued to exist, with slight modifications, up to the present 
time, and not less than one-tenth ol all the known species 
of fossil Echini belong to this important genus, which in 
our tropical seas is still a prominent one. It is interesting 
here to note that in the Cidarid® the. modifications of the 
test are not striking, and the fossil genera appearing in the 
successive formations are distinguished by characters which 
often leave us in doubt as to the genus to which many 
species should be referred. In the genus Rhabdocidaris, 
which appears in the lower Jura, and which is mainly char- 
acterized by the extraordinary development of the radioles, 
we find the extreme of the variations of the spines in this 
family. From that time to the present day, the most strik- 
ing differences have existed in the shape of the spines, not 
only of closely allied genera, but even in specimens of the 
same species ; differences which in some of the species of 
to day are as great as in older geological periods. The 
oldest Cidarid® are remarkable for their narrow poriferous 
zones. It is only in the Jura that they widen somewhat ; 
subsequently the pores become conjugated, and only later, 
during the Cretaceous period, do we find the first traces of 
any ornamentation of the test (Temnocidaris) so marked at 
the present day in the genus Goniocidaris. As far, then, 
as the Cidarid* are concerned, the modifications which 
take place from their earliest appearance are restricted to 
slight changes in the poriferous zone and in the ornamen- 
tation of the test, accompanied with great variability in the 
shape of the primary radioles. We must except from this 
statement the genera Diplocidaris and Tetracidaris, to 
which I shall refer again. The representatives of the other 
Triassic family become extinct in the lower tertiaries. The 
oldest genus, Hemicidaris, undoubtedly represents the 
earliest deviations from the true Cidaris type ; modifica- 
tions which affect not only the poriferous zone, but the test, 
the actinal and the abactinal systems, while from the extent 
of these minor changes we can trace out the gradual devel- 
opment of some of the characteristics in families of the reg- 
ular Echini now living. The genus Hemicidaris may be 
considered as a Cidaris in which the poriferous zone is 
narrow and undulating, in which the granules of the ambu- 
lacral system have become minute tubercles in the upper 
portion of the zone and small primary tubercles in its actinal 
region, in which many of the interambulacral granules be- 
come small secondaries, in which the plates of the actinal 
system have become reduced in number, and the apical 
system has become a narrow ring, and finally in which the 
primary radioles no longer assume the fantastic shapes so 
common among the Cidaridae. 
We can trace in this genus the origin of the modifications 
of the poriferous zone, leading us, on the one side, through 
genera with merely undulating lines of pores to more or 
less distinct confluent arcs of pores, formed round the 
primary ambulacral tubercles, and, on the other, to the 
formation of open arcs of three or more pairs of pores. 
The first type culminates at the present day with the Arba- 
ciadae, the other with the Diadematidae, Triplechinidae, and 
Echinometrad®. This specialization very early takes place, 
for already in the lower jura Stomechinus has assumed the 
principal characteristics of the Triplechinidae of to-day. 
Although in Hemicidaris the number of the coronal 
plates has increased as compared with the Cidarid*, and 
while we find that in many genera, even of those of the 
present day, the number of the coronal plates is still com- 
paratively small, yet, as a general rule, the more recent 
formations contain genera in which the increase in number 
of the interambulacral plates is accompanied by a corre- 
sponding decrease in the number of plates of the interam- j 
bulacral area so characteristic thus far of the Cidaridae and I 
Hemicidaridae, a change also affecting the size of the pri- 
mary ambulacral tubercles. This increase in the number 
of the coronal plates is likewise accompanied by the devel- 
opment of irregular secondary and miliary tubercles, and 
the disappearance in this group of the granular tubercula- 
tion, so important a character in the Cidaridae. With the 
increase in the number of the interambulacral coronal 
plates, the Pseudodiadematidae still retain prominent pri- 
mary tubercles, recalling the earlier Hemicidaridae and Cida- 
ridae, and, as in the Cidaridae proper, the test is 'frequently 
ornamented bv deep pits or by ridges formed by the junc- 
tion of adjoining tubercles. The genital ring becomes 
narrower, and the tendency to the specialization of one of 
its plates, the madreporite, more and more marked. 
With the appearance of Stomechinus, the Echinid® 
proper already assume in the Jura the open arcs of pores, 
the large number of coronal interambulacral plates, the 
specialization of the secondary tubercles, and the large 
number of primary tubercles in each plate. With the ap- 
pearance of Sphserechinus in the early Tertiary come in all 
the elements lor the greater multiplication of the pairs of 
pores in the arcs of the poriferous zones, while the gigantic 
primary spines of some of the genera (Heterocentrotus), 
and the small number of primary tubercles are structural 
features which had completely disappeared in the group 
preceding the Echinometrad®, to which they appear most 
closely allied. 
Going back again to the Hemicidarid®, it requires but 
slight changes to pass from them to Acrosalenia and to the 
Saleni® proper ; the latter have continued to the present 
day, and have, like the Cidarid®, retained almost un- 
changed the characters of the genera which preceded them, 
combined, however, with a few Cidaridian and Echinid fea- 
tures which date back to the Triassic period. We can thus 
trace the modifications which have taken place in the pori- 
ferous zone, the apical and actinal systems, the coronal 
plates, the ambulacral and interambulacral tubercles, as 
well as in the radioles, and in the most direct manner pos- 
sible indicate the origin of the peculiar combination of 
structural features which we find at any geological horizon. 
On taking in succession the modifications undergone by 
the different parts of the test, we can trace each one singly, 
without the endless complication of combinations which 
any attempt to trace the whole of any special generic combi- 
nation would imply. 
Leaving out of the question for the moment the Pal*chi- 
nid®, we find no difficulty in tracing the history of the char- 
acters of the genera of the regular Echini which have 
existed from the time of the Trias and are now living, pro- 
vided we take up each character independently. Nothing 
can be more direct than the gradual modification of the 
simple, barely undulating poriferous zone, made up of 
numerous ambulacral plates covered by granules, such as 
we find it among the Cidarid® of Trias, first into the 
slightly undulating poriferous zone of the Hemicidarid®, 
next into the indistinct arcs of pores of the Pseudodiade- 
matid®, then into the arcs with a limited number ol pores 
of the Triplechinid®, and finally to the polyporous arcs of 
the Echinometrad®. What can be more direct than the 
gradual modification to be traced in the development of the 
primary ambulacral tubercles, such as are characteristic of 
the Echinid® of the present day, from their first appearance 
at the oral extremity of the ambulacral system of the He- 
micidarid®, and the increase in the number of primary in- 
terambulacral tubercles, accompanied by the growth of 
secondaries and miliaries, which we can trace in Hemicida- 
ridis, Acrosalenia, and Stomechinus, — the increase in 
number of primary and secondary tubercles being accom- 
panied by a reduction in the size of the radioles and a 
greater uniformity in their size and shape ? 
But while these modifications take place, the original 
structural feature may be retained in an allied group. Thus 
the Cidarid® retain unchanged from the earliest time to the 
present day the few primary tubercles, the secondary gran- 
ules, the simple poriferous zone, the imbricating actinal 
system, and the few coronal plates, with the large apical 
system and many-shaped radioles; while in the Salenid® 
the primary internambulacral tubercles, the secondary 
granules, the radioles, the genital ring, are recognized fea- 
tures of the Cidarid®, associated, however, with an Echinid 
actinal and anal system, Hemicidarid primary ambulacral 
