228 REVIEWS 
Recognizing the radials as practically a single plate in each ray, all 
plates above must be regarded as brachials to which pinnules may be 
attached. The terms costals, distichals and palmars are appropriately 
applied to the first, second and third orders of brachials respectively. 
When there are further divisions in the rays, the plates are designated 
as postpalmars, or as brachials of the fourth and fifth orders, and so 
on. A discrimination is also made between fixed and free brachials, 
the latter often being termed the arms. ‘The arms are composed of one 
or two rows of plates. All biserial arms are uniserial in the young 
crinoid and gradually enter the biserial stage by an interlocking of the 
joints from opposite sides. In most of the families belonging to the 
Camerata the uniserial type is restricted to the Silurian, except in 
Hexacrinite. Among the Inadunata biserial arms occur only in a few 
genera found in the Kaskaskia, in the Coal Measures and in the Trias, 
but associated with the forms having the uniserial type. All Articulata, 
paleeozoic as well as neozoic have uniserial brachial appendages. 
The pinnules in a general way are repetitions of the arms on a 
small scale. When represented they spring alternately on opposite 
sides from every second joint and every joint bears a pinnule except 
in cases of a syzygy, in which the syzygyial plates must be counted in 
the alternation of the pinnules as one ossicle. Syzygies occur among 
Paleozoic crinoids either in successive series throughout the arm, as in 
the Heterocrinidz and Belemnocrinidz, or there is but one syzygy to 
each order of brachials, formed by the two proximal plates, as in Poter- 
iocrinus, Dichocrinus, and in most species of Platycrinus. In Dicho- 
crinus the various orders of brachials to the last axillary consist of two 
plates each, the first non-pinnulate, the upper bearing an arm instead 
of a pinnule. A similar arrangement occurs above the costals in most 
species of Platycrinus and it is quite evident that the plates in question, 
as in Dichocrinus for example, do form a syzygy. ‘This, however, is 
not the caseis such forms as Platycrinus huntsville and a few other species. 
Here the first pinnule is given off from the proximal distichal, and the 
second on the same side from the first palmar. It shows clearly that 
the arm partakes of the alternation of the pinnules, and suggests that 
the armlets are enlarged pinnules. This is shown more conclusively 
by the structure of Glyptocrinus dyert. While in most species of Glyp- 
tocrinus the second bifurcation takes place from the second distichial, 
that plate in G. dyer? gives off in place of an arm a large pinnule, 
more than twice as large as an ordinary one, which bending outward 
