A MONOGRAPH OF THE EXISTING CRINOIDS 39 



MOVABLE LIGAMENTARY UNIONS IN THE COMATULIDS 



Gisl6n noted that although the degree of flexibility is very differently developed 

 in different forms, nevertheless the synarthrial type taken as a whole is generally 

 the same. Cryptosynarthries may appear in the Comasteridae. In these, as in 

 pseudosyzygies and in the usual syzygies, the flexibility is practically nil. 



As viewed from the dorsal side synarthrial articulations show a fair amount of 

 variation. The articular line may be straight and not accompanied by any process; 

 there may be a more or less strongly developed synarthrial tubercle; or there may be 

 a more or less strongly developed synarthrial backward projection. 



Gisl6n said that it is important to differentiate between synarthrial tubercles 

 and synarthrial backward projections. In the former only a dorsal prolongation of 

 the synarthry occurs; this causes an enlargement of the synarthrial faces, but gen- 

 erally no increased flexibility worth mentioning, as the synarthrial fibers are not 

 elongated. To the latter type, on the other hand, great possibilities as regards 

 increased flexibility are always attached, with long ligamentary fibers in large lateral 

 notches between the ossicles. 



There is, as a rule, veiy little flexibility in the Oligophreata from the Comasteridae 

 to the Calometridae. Among these the backward synarthrial projection is very 

 slight or lacking. On the other hand in the Macrophreata, together with most of 

 the Thalassometridae and Charitometridae, the synarthries permit especially strong 

 lateral flexion, and the backward synarthrial projections are often enormously de- 

 veloped. These reach their maximum in the Zenometrinae and Bathymetrinae in 

 the family Antedonidae. Extraordinarily strongly developed synarthrial tubercles 

 appear sporadically in different families, for instance in species of Amphimetra, in 

 Neometra diana, and in Perometra diomedeae. 



Gisl6n said that in the youngest stages the synarthries tend toward synostoses, 

 but a stout areola appears very early. 



Another type of mobile articulation, trifascial ligamentary union or ligamentary 

 articulation, with dorsoventral flexion only, is found in some cases in the recent co- 

 matulids. This appears only in the articulation between the radials and the IBri. 

 It has been observed in Pontiometra andersoni and in Stephanometra spicata. Stand- 

 ing very near this type is the radial articulation in Himerometra martensi in which 

 the muscles are indicated only as rudiments in the shape of low bands. In the 

 Comasteridae and Calometridae also there is a form of articulation between the 

 radials and the IB^ that approaches this type. This type of trifascial articulation, 

 according to Gislen, possesses its greatest interest in that several of the Jurassic 

 comatulids seem to have a similar articulation between the radials and the IBr t . 



Comasteridae. — From a pronounced synarthrial type the articulation seems 

 sometimes to approach the cryptosynarthry, observed among the slender armed 

 varieties of the species of Comatula. The species studied by GislSn have fairly typical 

 synarthries. Divergencies from this have been observed in the synarthry between 

 the elements of the IBr series. In large species, such as Comanthus japonica, the 

 areola around the lumen is very extensive, and the two ligament pits are not equally 

 curved, but are excavated irregularly, here and there. Here the possibilities of 

 flexion are slight. The same is the case in those species of Comaster in which the 

 synarthries are replaced by pseudosyzygies. The small species of Comissia, as well 



