24h The American Naturalist. [March, 
manifest result of a much better defined subdivision of the series 
than that now existing. 
Geologic Distribution of the Actinocrinide-—More than three- 
fourths of the total number of genera of Actinocrinide are repre- 
sented in America, distributed in time as shown in the accom- 
panying chart, the relative expansion of each genus being also 
indicated. As compared with the ages preceding, the lower 
' Carboniferous is here greatly exaggerated in order to show more 
clearly the relationship of the several zoological groups, for it 
was during this time that the greatest diversity of form, structure 
and general ornamentation occurred ; in fact it was the culminat- ` 
ing period of crinoidal life in America. Continuous lines are 
drawn where the record is complete and the transitions fully 
shown, while the dotted lines indicate the relation of the different 
types according to the evidence at present known, and probably 
coincide very closely with the real courses of divergence. The 
scheme is, then, to represent in a graphic way the relationship of 
the genera as now understood, rather than to construct a genea- 
logical tree, with which attempts of this kind are often con- 
founded. In the present instance some of the earlier, more 
generalized forms have not as yet been made known. ‘There are 
also good grounds for believing that some of the generic types 
are considerably older than actual observation shows. In other 
groups, more particularly, there is abundant evidence pointing to 
a much higher antiquity of the leading generic types than is 
generally supposed. This is especially true of many widely 
distributed living toot whose ancestry has lately proved to 
be very ancient. 
Elements of Classification—The most generalized type of the 
family Actinocrinide has dorsally a single ring of basal plates, 
three in number and of equal size, succeeded by a second circle 
of subequal pieces, six in number—the five radial and the 
primary anal plates. As in all camarate crinoids, the brachials 
for a considerable distance are incorporated into the calyx by 
means of interradial ossioles, and in the free portion of the rays 
they are biserial and closely interlocking. Ventrally five orals 
can, with a few exceptions, be made out; they are usually sur- 
