SAGEN0CRIN1DAE 



239 



lAx 



I Br, 



brach ; thence it continues about the same to the tertibrachs, beyond which the plates gradu- 

 ally become thinner; from the last level upward there is little difference between the two 

 genera, but below it the plates of Forbesiocrinus are generally only about half as thick pro- 

 portionally as those of Taxocrinus. The best mode of expressing this is in the ratio of 

 width of the plates — representing the circumference of the calyx — to their thickness. The 

 average of measurements of several radials and axillary primibrachs of Forbesiocrinus 

 nobilis and agassisi in which the relative dimensions are similar, compared with Taxocrinus 

 colletti, shows the following ratios of width to thickness : 



Forbesiocrinus R : 1 to 0.3 ; iAx : 1 to 0.47 



Taxocrinus " 1 to 0.6; " 1 to 0.75 



These relations may be better shown by the fol- 

 lowing series of figures made from the radial and two 

 primibrachs of F. nobilis and T. colletti, giving the 

 dorsal outline together with a proximal view showing 

 the full thickness of the plates. Those of F. nobilis 

 are natural size ; in the Taxocrinus the dorsal outline 

 is natural size, while to facilitate comparison the 

 proximal view is drawn to the scale of the Forbesio- 

 crinus plates. In this way the great difference between 

 them can be readily seen. The relative thickness of the 

 plates in these forms as far as the highest tertibrach 

 may be seen in the figures on Plate XXI, figures 1, 2, 

 4; F. saffordi (fig. 3) being exceptional. This fact is 

 of some practical importance, as it will often facilitate 

 identification of the genera from fragmentary speci- 

 mens, or even isolated plates. 



Forbesiocrinus exhibits an extreme development 

 of the sinuous articulation in the brachial series which 

 has given rise to the idea of " patelloid plates " (see 

 Pis. XXII-XXIX). There is a large curved indenta- 

 tion at the middle of the distal margin of each brachial, 

 corresponding to a projection or process from the 

 proximal margin of the succeeding plate, which in a 

 moderately extended position of the ray perfectly fits 

 and fills it. Where the two are in that condition the 

 suture line between the plates, as visible externally, 

 has a double curve from the middle each way, produc- 

 ing a " wavy," " sinuous," or " arcuate " outline 

 (PI. XXIV, fig. 17a). This was observed by 

 De Koninck and Le Hon, and carefully noted in their 

 generic description of Forbesiocrinus. They thought 

 it a remarkable structure, such as had never been 

 observed in any other genus of crinoids. Hall 



(Geology of Iowa, vol. 1, pt. 2, p. 630) described and figured it more fully from a specimen 

 of F. agassisi, in which it is especially conspicuous, commenting as follows : 



The indentation on the upper margin does not extend throughout the thickness of the plate, but only 

 to a very moderate depth, and is filled by a superficial plate which is separately articulated and sometimes 

 anchylosed to the outer margin of the plate above, and lying over the suture below, somewhat like the 

 patella of the knee-joint. This patelloid plate is sometimes large. In many instances there is no suture- 

 line visible at the junction of the small patelloid plate with the next above; while in others, the suture is 



R 



Figs. 29-34 



Showing relative thickness of calyx 

 plates; Radial (RJ, first (IBn) and 

 third (IAx) primibrachs in natural order 

 from bottom up ; a, dorsal view ; b, prox- 

 imal view; 29, 30, 31, Forbesiocrinus 

 nobilis, all natural size; 32, 33, 34, Tax- 

 ocrinus colletti; a, a, a, natural size; 

 b, b, b, enlarged to same size as corre- 

 sponding plates of Forbesiocrinus. 



