298 Agassiz—Paleontological and Embryological Development. 
modifications which take place from their earliest appearance 
are restricted to slight changes in the poriferous zone and in 
the ornamentation of the test, accompanied with great varia- 
bility 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 
specialization very early takes place, for already in the lower 
Jura Stomechinus has assumed the principal characteristics of 
d 
development of irregular secondary and miliary tubercles, and 
the disappearance in this group of the granular tuberculation, 
