BY F, RATTE, ENG. ARTS AND MANUP., PARIS. 1163 



perpendicular to the straight articulation and passing at or near 

 the centre of the Calyx. It was, therefore, acting like a hinge. 



The shape of this articulation with the striated surfaces of 

 attachment of the muscles is represented Plate 68, figs. 2 to 

 12, on a doubled scale. 



It is nearly that of an isosceles triangle, the larger base of 

 which is the straight edge. The opposite obtuse angle is pro- 

 vided with an inner groove, which is the continuation of the 

 arm-channels or ambulacral groove, communicating with the 

 digestive apparatus. 



The upper side is convex, except round the groove, where it is 

 hollowed in the shape of a saucer to receive the next arm-plate. 

 Moreover, this saucer-shaped hollow is provided with a semi- 

 circular j!?«(^ ( " bourrelet ") or ridge, nearly concentrical with 

 the outer margin of the hollow, as seen in the genera Platycrinus 

 and Poteriocrinus, on the fixed plates that Prof, de Koninck calls 

 " pieces superieures " (1.) 



Arms. 



Traces of the arms are impressed above the last-mentioned 

 articulation, but not in a sufiicient state of neatness for descrip- 

 tion. I have seen in the collection of the Geological .Department a 

 beautiful impression, representing two branches of arms four 

 inches in length and i;,ths of an inch in thickness. Each article 

 is cuneiform in shape, say presenting a maximum of thickness at 

 the extremity of one diameter, and a minimum at the other 

 extremity of the same diameter alternately, the thickest part 

 (distal end) giving insertion to a spine. Some syzygies also are 

 distinguished. But it is not known if these arm-branches are 

 those of Trihrachyocrinus or of Cyathocrimis Konincki, which 

 both occur in the same beds. 



" VouTE " (Vault), or Outer Part of the Calyx. 



Between the arm-plates are seen the casts of very small plates, 

 irregular in shape, which doubtless belong to the so-called 

 "voute" (vault), or outer part covering the calyx, as in 

 Rhodocrimcs, for instance. Many of these small plates are four- 

 sided, few are five-sided. 



(1.) Desc. des Anim. Foss. Carb. Belg., 1842-4. Plate F. 



