882 Agassiz Paleontological and Embryological Development. 
seek for the origin of many structural features in types of 
which we have no record, or else we must attempt to fin 
them existing potentially in groups where we had as yet not 
succeeded in tracing them. The parallelism we have traced 
does not extend to the structure as a whole. What we find is 
3 he 
These are all structural features which are modified independ- 
ently one of the other; we may find simultaneous develop- 
ment of these features in parallel lines, but a very different 
degree of development of any special feature in separate fam- 
ilies. 
This is as plainly shown in the embryological as in the 
paleontological development. In the Cidaride there is the 
minimum of specialization in these structural features. In the 
ence of the few larger primary ambulacral tubercles at the base 
of the ambulacral area, and by their Diademopsid and Eehinid- 
