174 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Chadwick supposes that a primitive single brachial may develop a transverse 

 fracture plane which eventually becomes a syzygy, just as the peculiarly weakened 

 joints in the legs of crabs are developed, the similarity of which to crinoidal 

 syzygies was first called attention to by Bather in 1895. Although by this 

 explanation the morphological effect of the syzygy upon the arm structure would 

 be accounted for, it is definitely and emphatically negatived by the ontogeny of 

 the brachials. 



The only explanation of the syzygial pairs which accounts for all of their 

 many peculiar features is that they are strictly homologous with the first brachial 

 pairs of the arms, and with the pairs of ossicles in the division series, and represent 

 pairs of ossicles in every way comparable to these which have remained unmodified 

 during the course of the development of the immensely elongated and highly 

 specialized arm of the later crinoid types. 



Pseudosi/zi/gy. 



In the great majority of the comatulids the first syzygy is between the third 

 and fourth brachials of the free undivided arms, or between the reduplications 

 of these ossicles in the division series when the latter are composed of three or 

 four elements. But in a few types one or more of the synarthries which occur 

 proximal to the normal first syzygial pair are replaced by more or less perfectly 

 developed syzygies (part 1, figs. 37-40, p. 75) . On account of the anomalous position 

 of these syzygies in the comatulid arm and the fact that usually they diifer more 

 or less widely from the distal syzygies in the direction of the synarthries they 

 have been differentiated from the brachial syzygies under the name of pseudo- 

 syzygies. 



It should be emphasized that in reality there is no morphological difference 

 between the syzygies as found from the second brachial pair onward and the 

 syzygies that replace the more proximal synarthries. The latter show all possible 

 intergradations from the perfect syzygy indistinguishable from that in the free 

 arms to a syzygy which is merely a slightly modified synarthry. In other words, 

 the transition in these proximal syzygies between the true syzygy and the typical 

 synarthry is complete and unbroken, showing that there is no fundamental differ- 

 ence between syzygies and synarthries and that all the postradial nonmuscular 

 articulations are really homologous morphologically as they are histologically. 



The proximal syzygy reaches its greatest perfection in the species of the family 

 Zygometridse, where it occurs only between the elements of the IBr series, all of 

 the other division series being either 2 or 4(3-|-4) as usual and the first two 

 brachials of the free undivided arms being united by synarthry. 



In the Zygometridse the proximal syzygy is quite indistinguishable from a 

 brachial syzygy excepting only in a single small 10-armed species, Zygometra 

 pristina, in which the outer part of the articular faces for about one-half of the 

 distance from the periphery to the rim of the central canal is marked with radiat- 

 ing ridges, the remainder of their surface being smooth and flat except for the 

 low and narrow median synarthrial ridge. 



