20 



SMITHSONIAN INSTITUTION 



distal facets of the basals have single, deep, inwardly sloping fossse (text- 

 fig, n). The proximal facets of the radials have single, very deep fossae 

 corresponding in outline to those of the basals, but with their floors divided 

 into a lateral series of conical pits from which the crenulae radiate (text- 

 fig. 12). The facets of the primibrachs, lower secundibrachs, and inter- 

 brachials show still further modification, the walls of the pits having thickened, 

 and increased in height until they came into contact with similar walls in the 

 apposed facets. The contact areas between the upper portions of the small 

 ligament pits are usually deeply lobed by the depression of the surface along 

 the plate margins, thus forming a series of external pits between the plates, 

 and leaving the ligament fossae surrounded by narrow contact bands which 



Mode of union between plates in Scyphocrinus. II, distal face of BB. 12, proximal face 

 of R. 13, distal face of IBr. 14, distal face of higher IIBr. 15, 16, distal and lateral faces 

 of IIIBr. All X4- 



project outwardly at right angles from the internal marginal band as a series 

 of loops (text-fig. 13). The facets of the higher secundibrachs have single, 

 deeply inward sloping fossae surrounded by a broad, smoothly outlined mar- 

 ginal contact band (text-fig. 14), but in the tertibrachs the fossae become 

 very shallow, merging imperceptibly with the contact band of the outer margin, 

 and in some examples completely surrounding the plates near the inner margin 

 of the inner contact band (text-figs. 15, 16). 



This is a type of loose suture formed largely by interlocking crenulations 

 admitting slight mobility, similar to that between the usual discoid stem ossicles, 

 associated with a more or less pliant tegmen of undifferentiated small plates, 

 occurring in forms like Reteocrinus and Glyptocrinns, which exhibit variation 

 from the Camerate calyx toward that of the Flexibilia. 



The internal surface of the calyx plates of Scyphocrinus is ornamented by 

 fine striae arranged at right angles to the sutures, forming trapezoidal areas 



