1890.] Genesis of the Actinocrinide. 253 
form the radial portions. of the calyx have already commenced to 
become somewhat lobate, and the arms to grow longer and more 
slender. The plates of the ventral side are all quite small, the 
ovals indistinguishable from the surrounding ossicles; while the 
_ pieces around the anal tube are still smaller, indicating that this 
structure was very short, and in many cases probably did not pro 
ject much above the ventral dome. The ornamentation of both 
also presents a close similarity. Some forms of A. orpusculus 
Hall, from the lower part of the Burlington limestone, also show 
the Physetocrinus facus, but in a much less merked degree 
Physetocrinus appears to be the line along which Strotocrinus 
developed into the unique, short-lived forms which are found only 
in the upper part of the Burlington. 
With the calyx alone under consideration Steganocrinus 
would be immediately referred to Actinocrinus, but the immense, 
narrow, radial extensions from which spring the free arms are 
certainly distinctive enough for generic separation. Although in 
this character the genus, at first sight, departs so far from the 
other groups of the family, it will be seen on closer examination 
that the departure is only another phase of what is shown in 
Strotocrinus, a devergence beginning a little earlier and in a little 
different direction. 
Summary. Several interesting points are disclosed in the fore- 
going sketch of the American Actinocrinide, which have an im- 
portant bearing upon the consideration of fossil faunas in general. 
The conclusions here arrived at apply, in the main, to other 
families of crinoids as well as to the gastropods and other paleo- 
zoic forms, though not to so apparent an extent. Considerable 
information has already been obtained illustrating these phe- 
nomena in the groups last mentioned, and will be the subject of 
future reference. 
(1.) It is clearly indicated that a large proportion of the genera 
date back much further geologically than actual observation 
shows. 
(2.) At times in the phylogenetic history of a group varia- 
tions appear to go on with broad and rapid strides, and the 
organisms survive through rapidly changing physical conditions. 
