A MONOGRAPH OF THE EXISTING CRINOIDS 41 



tubercles are found. This is often developed in connection with longitudinal crests 

 on the proximal brachials. There is often a synarthrial backward projection, and 

 at the same time large notches between the ossicles, giving the capacity for increased 

 lateral flexion. The appearance of the synarthrial face is much like that in the follow- 

 ing family. 



Charitometridae — The synarthrial faces in this family correspond in many ways 

 to those in the family preceding. There is often a longitudinal crest on the proximal 

 ossicles, and sometimes synarthrial backward projections. There are often large 

 lateral notches. The synarthrial face often shows a likeness to that of certain ante- 

 donids in being restricted to a rather small proportion of the joint face. 



Notocrinidae. — The synarthrial face reminds one of that in the Thalassometridae. 

 The median dorsoventral ridge is, however, extraordinarily stout, the areola surround- 

 ing the lumen very broad, and the ligament fossae deep. There are no synarthrial 

 tubercles. There is a weak synartlnial backward projection. 



Antedonidae. — Great variation is found in this family. Gislen said it looks as 

 though one might trace the appearance of the synarthrial faces back to two types. 

 The first of these is like that in Asterometra and Notocrinus in that the articular facet 

 occupies practically the whole of the joint face, and the synarthrial backward projec- 

 tion is moderately developed; there are rarely any synarthrial tubercles (but these 

 are strongly developed in the Perometrinae). The Antedoninae belong here, and also 

 the Perometrinae and Isometrinae and probably the Thysanometrinae. The second 

 type has the synarthrial facet restricted to the center of the joint face and beyond 

 this a broad area without ligament fibers; the synarthrial facet is often produced 

 dorsoventrally, and the backward synarthrial projection is generally enormous. Here 

 belong the Heliometrinae, Zenometrinae, and Bathymetrinae. 



Pentametrocrinidae. — The synarthrial facet occupies practically the whole of the 

 joint face, but relatively somewhat less in Pentametrocrinus. It is narrower in Th.au- 

 matocrinus, evidently because of the 10 arms. The fossae are moderately deep. The 

 synarthrial backward projection is distinctly pronounced in Pentametrocrinus, but 

 imperceptible in Thaumatocrinus. 



Atelecrinidae.— The synarthrial facet is very much like that in the Zenometrinae. 

 The synarthrial backward projection is distinct. 



ARTICULATIONS OF THE PINNULARS IN RECENT/CRINOIDS 



Gislen said that on the whole the pinnular articulations seem to be very uni- 

 formly constructed in all the recent crinoids, though there are wide variations in 

 several details. The typical form may be described as follows. 



The articulation between a brachial and the first pinnule is an articulation of 

 rather ordinary muscular type. While the angle between the transverse ridges of the 

 two joint faces on the distal end of an axillary is about 90° (for instance in Heliometra 

 and Himerometra—the axillary being viewed from the distal end), the transverse 

 ridges of the pinnule facet and of the brachial joint face form a considerably greater 

 angle usually (Heliometra 115°, Metacrinus 150°, Himerometra about 165°, and Calo- 

 metridae and other types 180°). The pinnule facet on the brachial is thus twisted 

 nearly halfway around, so that the side that should be the outer lateral becomes the 



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