MORPHOLOGICAL PART. 81 
mouth. In their normal condition the pinnules are arranged alternately on 
opposite sides from every second joint, so that each joint bears a pinnule. 
When the interval between the pinnules is greater, which is frequently the 
case at certain parts of the arms, especially among recent Crinoids, this has 
been called a syzygy. The term was applied by Johannes Miiller* to 
the immovable union of two arm joints, of which only the upper one is 
pinnule-bearing. The two plates, to which the names “hypozygal” and 
“ epizygal” were given, — the later for the pinnule-bearing one, — count in 
the alternation of the pinnnles as one joint. Their apposed faces are not 
necessarily striated, as indicated by Miiller, being sometimes dotted or 
smooth, Smooth syzygial faces have been observed in recent Crinoids only 
in a few Comatulee, but they occur quite frequently among Paleozoic species. 
In the latter, we also find occasionally a succession of two or three hypozygal 
joints in the same syzygy.T 
Syzygies do not occur among Paleozoic Crinoids at irregular intervals as 
in recent ones. Either they have a continuous series of syzygies through- 
out the whole arm, as in the Heterocrinide and a few genera of the Came- 
rata, or there is one syzygy in each order of brachials, which is formed 
by the two proximal plates. The latter is frequently the case among the 
Poteriocrinide, Encrinidz, and also in the Camerate genus Dichocrinus. In 
Dichocrinus, the suture between the syzygial plates is very close, and the line 
of union more faint than the lines of adjoining plates. In the Poteriocrinidz 
either the apposed surfaces are flat, or the hypozygal plate is slightly ex- 
cavated, and the epizygal correspondingly convex. In Hnerinus lihifornus the 
corresponding faces are dotted so as to make a very close union. The Poterio- 
crinidz may have one or two costals within the same genus. In species with 
two, both plates together take the form and size of the one; and this rule 
applies to the proximal distichals as well. That the two plates of the costals, 
as well as those of the distichals, form a syzygy, is practically shown by the 
pinnules, which in species with but one costal begin with the first plate after 
* Ueber den Bau des Pentacrinus caput meduse (Abhandl. d. K. Akademie der Wissenschaften, Berlin, 
1843, p. 215). 
+ The term “ syzygy” has also been used by some writers for the immovable union of the nodal stem- 
joints with those next below them, as in the case of Peztacrinus. This, we think, is not in accordance with 
Miiller’s definition, who proposed the term for two arm-joints of which only the upper is pinnule-bearing, 
and not for a special mode of union between plates generally. Radiated and dotted surfaces do not always 
imply a syzygy. Such faces are found among Paleozoic Crinoids very frequently on the ordinary arm plates, 
and even, as in Crofalocrinus, between the plates of the dorsal cup. ‘The union between the syzygial joints is 
nothing but an ordinary close suture, which may have striated or smooth surfaces. 
II 
