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Agassiz— Paleontological and Embryological Development. 388 
ian affinities that we explain the indented imbricated actinal 
system with the presence of a few genuine miliaries. But all 
the structural features which characterize the earliest types of 
spines of the abactinal region of the test naturally leads to 
similar spines covering the whole test in the other families of 
the Desmosticha. The .difference existing in the plates cover- 
less gibbous forms, next that of the Laganide, and finally of 
the flat Scutellides; while we trace in the Echinanthide the 
Spatangoid genera, the modifications of a test in which the 
ambulacral and interambulacral areas are made up of plates of 
nearly uniform size, in which the anterior and posterior extrem- 
ities are barely specialized, to the most typical of the Ananchy- 
tide, in which the anterior and posterior extremities have devel- 
oped the most opposite and extraordinary structural features. 
In a similar way we can trace among the fossil genera of differ- 
ent families the gradual development of the actinal plastron 
from its very earliest appearance as a modification of the posterior 
interambulacral area of the actinal side, or the growth of the 
posterior beak into an anal snout, the successive changes of the 
anal groove, the formation of the actinal labium, or the devel- 
