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 Mliller*^ 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 pinnules as one joint. Their apposed faces are not 

 necessarily striated, as indicated by MUller, being sometimes dotted or 

 smooth. Smooth syzygial faces have been observed in recent Crinoids only 

 in a few Comatulse, but they occur quite frequently among Palasozoic species. 

 In the latter, we also find occasionally a succession of two or three hypozygal 

 joints in the same syzygy.f 



Syzygies do not occur among PaloBozoic Crinoids at irregular intervals as 

 in recent ones. Either they have a continuous series of syzygies through- 

 out the whole arm, as in the Heterocrinidse 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 

 Poteriocrinidae, Encrinidae, and also in the Camerate genus Dichocriniis. In 

 DieJiocrinus, the suture between the syzygial plates is very close, and the line 

 of union more faint than the lines of adjoining plates. In the Poteriocrinidse 

 either the apposed surfaces are flat, or the hypozygal plate is slightly ex- 

 cavated, and the epizygal correspondingly convex. In Encrinus liliiformis the 

 corresponding faces are dotted so as to make a very close union. The Poterio- 

 crinidae 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 medusas (Abliandl, d. K, Akademie der Wissenscliaften, Berlin, 

 1843, p. 215). 



f The term " syzygy " has also been used by some writers for tlie immovable union of the nodal stem- 

 joints with those next below them, as in tlie case of Pentacrinus. 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. Eadiated and dotted surfaces do not always 

 imply a syzygy. Such faces are found among Palaeozoic Crinoids very frequently on the ordinary arm plates, 

 and even, as in Crotalocrinus, 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. 



