4 
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Agassiz— Paleontological and Embryological Development. 885 
sequent formations. In order to make our parallelism, we must 
go back to a stage in the embryonic history of the young 
Kchini in which the distinction to be made between the ambu- 
lacral and interambulacral systems is very indefinite, in which 
the apical system is, it is true, specialized, but in which the 
actinal system remains practically a part of the coronal system. 
But here the comparison ceases, and, although we can trace in 
the paleontological development of such types as Archxocida- 
ris or Bothriocidaris modifications which would lead us with- 
out great difficulty, on the one side to the Cidaridse, and on the 
other to the Echinothurise and Diadematide of the present 
day, we cannot fail to see most definite indications in some of 
the structural features of the Palechinide of characteristics 
which we have been accustomed to associate with higher 
groups. The minute tuberculation, for instance, of the Clype- 
astroids and Spatangoids, already existing in the Melonitide, 
as Cidarid. The polyporous genera of the group represent to 
a certain extent the polypori of the regular Echini, and the 
lapping of the actinal plates of the Cidaridze and of the coronal 
plates in some of the Diadematide, as well as the existence of 
such genera as Tetracidaris, of four interambulacral plates in 
Astropyga, and of a large number of ambulacral plates in 
some of the recent Echinometrade, all these are Paleechinid 
characters which we can explain on the theory of the inde- 
pendent development of the structural features of which they 
are modifications. We should, however, remember, that the 
existence of a large number of coronal plates, especially inter- 
ambulacral plates, in the Palechinide, is a mere vegetative 
character, which they hold in common with all the Crinoids,— 
a character which is reduced to a minimum among the Holo- 
thurians, and still persists in full foree among the Pentacrini 
of the present day, as well as the Astrophytide and Echinide. 
It would lead me too far to institute the same comparison 
between the embryonic stages of the different orders of Echin- 
are identical with the fossil orders, and that as far as we know 
they all begin at a stage where it would be impossible to dis- 
tinguish a Sea-urchin from a Star-fish, or an Ophiuran, or a 
Crinoid, or an Holothurian,—a stage in which the test, calyx, 
abactinal and ambulacral systems are reduced to a minimam. 
From this identical origin there is developed at the present day, 
in a comparatively short period of time, either a Star-fish, a 
Sea-urchin, or a Crinoid; and if we have been able success- 
Am. Jour. a Series, Vou. XX, No. 119.—Nov., 1880. 
