MONOGRAPH OF THE EXISTING CRIXOIDS. 213 



is a syzygy, the proximale, naturally taking the shape of the dorsal part of the 

 calyx, becomes pentagonal or circular and assumes the function of a central dorsal 

 plate. 



Now the enlargement of the proximale affects also the columnal just beneath it, 

 the proximal (iipper) face of which increases to a size equal to that of the distal 

 0ower) face of the proximale and, entirely losing the characteristic joint face 

 sculpture, becomes closely approximated t<> the distal face of the proximale, uniting 

 with it in exactly the same way as the proximale unites with the calyx plates. This 

 onion between the proximale and the columnal just below it is the so-called stem 

 syzygy; but it is in reality merely a close suture, strictly homologous with the close 

 suture between the proximale and the basals and between the basals and the radials. 

 Proximales, or columnals homologous to proximales, are always attached to the 

 columnals just below them by these so-called syzygies, which differ from the other 

 articulations of the stem in having a plane, or nearly plane, surface without radial 

 crenellse, petaloid sectors, or transverse ridges; in other words, resembling the 

 surface of the radials to which the centrodorsal is attached, or by which the radials 

 are attached to each other. 



Primarily there was but one syzygy in the column, that between the proximale 

 and the columnal just below it. Such an arrangement is seen in the pentacrinites, 

 in which the proximale is reduplicated at regular intervals along the stem in the 

 shape of so-called nodals, all of which are united to the infranodals by syzygy, and 

 in the comatulids, hi which the single stem syzygy is the seat of the fracture by 

 which the animal becomes free. 



The formation of the proximale, closely attached to the dorsal surface of the 

 calyx and fused with the infrabasals. prevents the formation of new columnals above 

 it and marks the maturity or end of growth of the stem. But columnal formation 

 may continue by intercalation between the columnals immediately below the stem 

 syzygy, or excessive vegetative power may shove the proximale outward before it 

 fuses with the calyx. In the adult pentacrinites new proximales are continu- 

 ally forming beneath the calyx, where every new columnal formed is a proximale, 

 only to be pushed outward by younger ones. Later these become separated by 

 intercalated segments, each of them becoming united by syzygy to the intercalated 

 segment immediately below it. 



In Rhizocrinus, Batlit/crinus and allied genera syzygies are found throughout the 

 column, with increasing frequency toward the crown. Each of the syzygial pairs 

 represents an effort to form a proximale, and each is the exact equivalent of the 

 nodal of the pentacrinite plus the infranodal (just beneath it); the enormous vege- 

 tative power of the column, though much less than in the pentacrinites. has pre- 

 vented the fixation of the proximale by the formation of added columnals above it, 

 while the more uniform growth has prevented its specialization, and the incipient 

 proximale. united to the columnal just below it, has passed outward in the shape of a 

 syzygial pair. 



The series of short discoidal columnals at the summit of the stem of Monachocri- 

 nus (figs. 132, 134, p. 203) and allied genera corresponds exactly to the cone-like struc- 

 ture at the summit of the stem in Apiocrinus. This latter has resulted through the 



