48 SMITHSONIAN INSTITUTION 



PLATE XXI 



PAGE 



A series of diagrams to illustrate the structural differences between Forbesiocrinus and the 

 leading Taxocrinoid genera ; and the mode of union of plates at the lateral margin of the rays 

 in both the regular and anal interradii. The same parts are represented in most of the diagrams, 

 viz : the right posterior and right anterior rays, with the basal plates supporting them ; and a, 

 the marginal face of r. ant. ray; h, the marginal face of r. post, ray; c, the distal margin of iBr. 



Fig. I. Forbesiocrinus nobilis. Mountain limestone. Shows: (l) Form of post. B, with sutural 

 distal face supporting anal plates articulated to adjacent rays. (2) Deep fossae around 

 BB, and at the brachio-interbrachial sutures of r. ant. ray ; with corrugated, straight, 

 thick marginal surface above them, and at distal margin of iBr, for attachment of 

 structure different from ordinary perisome. (3) Interbrachials with curved concave 

 distal margin, not extending above IBr 249 



2. Forbesiocrinus agassizi. Upper Burlington limestone. The same parts. Marginal faces of 



ray, angular for sutural attachment as high as upper IIIBr, and iBr extending in an 

 apex to about the same height 250 



3. Forbesiocrinus saffordi. Warsaw Gr. The same parts ; showing thicker plates, and closer 



union at sutural faces 259 



4. Taxocrinus colletti. Keokuk Gr. Shows: (i) Form of post. B, with plates of anal tube 



resting in socket below distal margin; (2)ff. Margin of r. ant. ray, with sutural face 

 for attachment of iBr up to IIBr i or 2, and smooth, rounded surface above for 

 attachment of perisome. (3)&. Margin of r. post, ray from R up, next to anal area, 

 with no sutural face, but smooth and rounded. (4)c. Distal face of iBr, similarly 

 smooth and rounded. Note that these smooth margins entirely lack the straight cor- 

 rugated surface seen in fig. i 398 



5. Onychocrinus ulrichi. Keokuk Gr. The same parts as in last figure ; the sutural face for 



iBr extends higher up 425 



6. Parichthyocrinus subovatus. Keokuk Gr. The same parts. Shows the same type of 



post. B, and anal area with smooth margins of adjacent rays, as in Taxocrinus, but 

 with arms interlocking above iBr, giving sutural faces high up on rays next to regular 

 interradii 415 



