252 eet The American Naturalist. [March, 
Actinocrinus is the type of a most remarkable group. The 
earlier forms bear a close resemblance to those of Batocrinus, but 
the possession of only two plates in the second anal tier serves 
readily to distinguish the two genera. As yet it has not been 
found to occur below the Carboniferous. It early shows a 
marked tendency to differentiation along the radial lines, assum- 
ing most wonderful phases, which culminated in Teleiocrinus, 
Strotocrinus, and Steganocrinus. The more primitive forms of 
Actinocrinus have the free arms, as they leave the calyx, nearly 
at equal distances from one another, In certain species, how- 
ever, the arms of one ray begin to separate from those of the 
adjoining rays. Interradial plates still further increase the dis- 
tance between the clustered free arm bases of the several rays, 
until finally the calyx has become strongly quinquelobate. The 
first section gradually diminished in numbers and disappeared in 
the upper part of the Burlington; but the second continually 
grows more and more prominent, and ultimately attains huge 
dimensions before the extinction of the group. 
_ In the upper portion of the Burlington appears a small group 
of crinoids—Teleiocrinus—possessing all the characters of Ac- 
tinocrinus except that the lower branchials for some distance 
have become larger, and appear like calyx plates. These are all 
firmly anchylosed, and do not give off the fine biserial arms until 
the fifth or sixth order of brachials. The calyx thus possesses a 
more or less well-defined lateral extension passing around above 
the branchial of the second order. This has led to the union of 
this group with Strotocrinus; but the rim, though very striking 
and very similar in each, appears to be a separate development in 
the two genera, rather than different stages of the same feature. 
In the ornamentation, the ventral structure, and the possession of 
a very long anal tube the affinities of Teleiocrinus are manifestly 
much nearer the typical representative of the family than Stroto- 
crinus. 
The Physetocrinus type begins to make its appearance in the 
Kinderhook as a derivative of Actinocrinus. The earliest known 
divergence, perhaps, is shown best in A. ornatissimus W. and 
Sp. from the lowest member of the lower Carboniferous. In this 

