Agassiz— Paleontological and Embryological Development. 301 
There is, however, not one of the simple structural features 
in the few types of the Triassic and Liassic Echini from whic 
we can so easily trace the origin of the structural features of 
all the subsequent Kchinid genera, which is not also itself con- 
tinued to the present day in some generic type of the present 
epoch, fully as well characterized as it was at the beginning. 
In fact, the very existence to-day of these early structural fea- 
tures seems to be as positive a proof of the unbroken system- 
atic affinity between the Echini of our seas and those of the 
Trias, as the uninterrupted existence of the genus Pygaster or 
Cidaris from the Trias down to the present epoch, or of the 
connection of many of the genera of the Chalk with those of 
our epoch (Salenia, Cyphosoma, Psammechinus, etc.). 
Passing to the Clypeastrids, we find there as among the 
Desmosticha that the earliest type, Pygaster, has existed from 
the Trias to the present time; and that, while we can readily 
reconstruct, on embryological grounds, the modifications the 
earliest Desmosticha-like Kchini should undergo in order to 
assume the structural features of Pygaster, yet the early peri- 
ods in which the precursors of the Echinoconide and Clypeas- 
tride are found have thus far not produced the genera in which 
these modifications actually take place. But, starting from 
Pygaster, we naturally pass to Holectypus, to Discoidea, to 
Conoclypus, on the one side, while on the other, from Holec- 
typus to Echinocyamus, Sismondia, Fibularia, and Mortonia, 
we have the natural sequence of the characters of the existing 
Kehinanthide, Laganide, and Scutellids, the greater number 
of which are characteristic of the present epoch. If we were 
to take in turn the changes undergone in the arrangement of 
the plates of the test, as we pass from Pygaster to Holectypus, 
to Echinocyamus, and the Echinanthide, we should have in 
the genera which follow each other in the paleontological record 
an unbroken series showing exactly what these modifications 
have been. In the same way, the modifications of the abacti- 
nal and anal systems, and those of the poriferous zone, can 
equally well be followed to Echinocyamus, and thence to the 
ypeastridze ; while a similar sequence in the modifications of 
these structural features can be followed from Mortonia to the 
Scutellidse of the present period. 
Passing finally to the Petalosticha, we find no difficulty in 
tracing theoretically the modifications which our early Echino- 
conidé of the Lias should primarily undergo previous to the 
appearance of Galeropygus. The similarity of the early Cassi- 
duloid and Echinoneoid types points to the same systematic 
affinity, and perbaps even to a direct and not very distant rela- 
tionship with the Palechinide. For if we analyze the Echino- 
thuriz of the present day, we find in genera like Phormosoma 
