Agassiz— Paleontological and Embryological Development. 379 
system in order to become petaloid, and the gradual change of 
the elliptical ovoid test of the young to the characteristic generic 
test, accompanied by the rapid increase in the number of the 
the simple actinostome to a labiate one, the specialization of 
the anterior and posterior parts of the test, and the definite 
formation of the fascioles. 
Comparing this embryonic development with the paleonto- 
logical one, we find a remarkable similarity if both, and in a 
general way there seems to be a parallelism in the appearance 
of the fossil genera and the successive stages of the develop- 
ment of the Hchini as we have traced it. 
We find that the earlier regular Echini all have more or less 
a Cidaris-like look,—that is, they are Echini with few coronal 
plates, large primary tubercles, with radioles of a correspond- 
ing size; that it is only somewhat later that the Diademopsidx 
make their appearance, which, in their turn, correspond within 
and miliaries, and traces of a Hemicidaris-like stage in the size 
of the actinal ambulacral tubercles. 
omparing in the same way the paleontological development 
of the Echinidx proper, we find that, on the whole, they agree 
well with the changes of growth we can still follow to-day in 
their representatives, and that, as we approach nearer the pres- 
ent epoch, the fossil genera more and more assume the struct- 
ural feats which we find developed last among the Echinidee 
of the present day. Very much in the same manner asa young 
Kchinus develops, they lose, little by little, first their Cidarid- 
ian affinities, which become more and more indefinite, next 
their Diadematidian affinities, if I may so call the young stages 
to which they are most closely allied, and, finally, with the 
increase in the number of the coronal plates, the great nu- 
fossil Echini of the Tertiaries, we pass insensibly into the 
generic types characteristic of the present day. 
__ Although we know nothing of the embryology of the Salen- 
id, yet, like the Cidaridw, they have in a great measure 
remained a persistent type, the modifications of the group being 
all in the same direction as those noticed in the other Desmos- 
