Agassiz— Paleontological and Embryological Development. 299 
so important a character in the Cidaride. With the increase 
in the number of the interambulacral coronal plates, the Pseu- 
dodiadematide still retain prominent primary tubercles, recall- 
ing the earlier Hemicideride and Cidaride, and, i 
Cidaridze proper, the test is frequently ornamented by deep 
and more marked. 
With the appearance of Stomechinus, the Echinide proper 
already assume in the Jura the open ares of pores, the large 
number of coronal interambulacral plates, the specialization of 
(Heterocentrotus), and the small number of Sree: tubercles 
y disappeared in 
the group preceding the Echinometrada, to which they appear 
most closely allied. 
Going back again to the Hemicidarida, it requires but slight 
changes to pass from them to Acrosalenia and to the Salenize 
proper; the latter have continued to the present day, and have, 
like the Cidaridaw, retained almost unchanged the characters of 
the genera which preceded them, combined, however, with a 
few Cidaridian me Kchinid features which date back to the 
riassic period. We can thus trace the modifications which 
have taken place in the poriferous zone, the apical and actinal 
systems, the coronal plates, the ambulacral and interambula- 
cral tubercles, as well as in the radioles, and in the most direct 
manner possible indicate the origin of the peculiar combina- 
tion of structural features which we find at any geological hori- 
zon. On taking in succession the modifications undergone 
the different parts of the test, we can trace each one singly, 
without the endless complication of combinations which any 
attempt to trace the whole of any special generic combination 
would imply 
