160 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



the radials and between the radials and the centrodorsal in the comatulids, and 

 also in the brachial and stem syzygies in the pentacrinites, or they may develop 

 more or less numerous irregularly meandering to regularly radiating ridges, as 

 in the unions between the calyx plates in many of the Flexibilia, and in the brachial 

 syzygies and so-called pseudosyzygies in the comatulid arms. A slight amount 

 of motion is usually possible between plates thus united. This type of union is, 

 when movable, known as the loose suture, and when almost or quite immovable as 

 the close suture. In the arms and in the column it is known as the syzygy. 



From this type of union there are two lines of departure — one in the direction 

 of greater rigidity, the other in the direction of greater flexibility. 



The close suture may become quite immovable and the ossicles united by it 

 inseparable by caustic through the development of a calcareous deposit between 

 and among the fibers, and this calcareous deposit may become so dense as to 

 result in welding the ossicles into a single unit in which no suture lines are visible. 

 Such a union is seen between the infrabasals and the centrodorsal in the coma- 

 tulids, and between the basals in Bathycrinus, llycnmas^ Rhizocrinus^ and certain 

 species of Ptilocrinus. 



The loose suture may become flexible, resulting in the development of a fulcral 

 ridge, the beveling away of the joint faces on either side of it, and the segre- 

 gation of the ligament fibers into two bundles, as is seen in the so-called synarthries 

 between the pairs of ossicles in the division series and between the elements of the 

 first brachial pairs, in the articulations between the pinnulars and cirrals, and in 

 the articulations in the so-called bourgueticrinoid type of column. 



In the arms when the fulcral ridge of a flexible articulation runs at right 

 angles to the dorsoventral plane the ventral ligament bundle is divided into two 

 more or less triangular bundles through the intrusion in the median dorsoventral 

 line of the articulation of two muscle bundles of extra-articular origin which 

 occupy a more or less triangular area the apex of which reaches nearly to the 

 central canal. 



Union of radials with each other and with centrodorsal, and union of calyx plates. 



No detailed histological study has as yet been made of the union of the radials 

 with each other and with the centrodorsal in the comatulids. 



P. H. Carpenter has described in considerable detail the characteristics of the 

 suture between the radials in Endoxocrinus wyvillethomsoni, which he states is of 

 the same character as that between the radials of the comatulids. 



He writes that in the immediate neighborhood of their apposed lateral faces 

 there are none of the nuclei nor pigment granules which are embedded so abun- 

 dantly in the more internal portions of their protoplasmic ground substance, and 

 the threads of the plexus of which it is composed become excessively attenuated 

 and disposed with great regularity almost parallel with one another. At the 

 same time the meshes of this organic plexus become greatly elongated in the 

 intervals between the parallel threads or fibers which are connected with one 

 another by very delicate fibrils passing in the form of loops from one fiber to 

 another. These loops are simply the expression of the ends of elongated meshes 



