Agassiz— Paleontological and Embryological Development. 381 
young Hemiaster, we now know to be a strictly embryonic 
type, which, while it thus has affinities with the true Spatan- 
goids, still retains features of the Cassidulide in the mode of 
the anal system. The genera following this group, Holaster 
and Toxaster, can be well compared, the one to the young 
Can trace an agreement which, as we go further back in time, 
becomes more and more limited. We are either compelled to 
