SAGENOCRINIDAE 229 



statement in a paper on the crinoid genera Sagenocrinus, etc. (American Geologist, vol. 30, 

 p. 91). This second specimen and a third one, figure 3, confirm the first in showing that the 

 posterior basal is followed by solid anal plates suturally joined to it — thus clearly separating 

 this species from Onychocrimis or any other Taxocrinoid. This character determines the 

 affinities of the genus to be with the present family, where it may be considered as a successor 

 of the Silurian Tcmnocrinus with the interbrachial characters of Sagenocrinus added in the 

 later American species. 



Instead of having the anal and interbrachial areas completely filled with solid plates 

 forming part of the calyx wall and extending to an apex high up between the rays, which 

 is one of the most characteristic features of the last mentioned series of species, these plates 

 in the Belgian species are limited to the lower part of their respective areas, having their con- 

 tinuous margin somewhat crescentic. By comparing the lateral margins of the rays in 

 specimens of the two types (Pis. XXIII, figs, la, c, d; XXV, fig. 2; XXI, figs. 1, 2, 3), it will 

 be observed that in F. agassizi and F. saffordi these interlock with the interbrachial plates by 

 angular faces and perfectly flat surfaces all the way up, producing a zig-zag suture line; 

 whereas in the Belgian form this interlocking ceases at about the angle of IBr„, and from 

 that point upward the lateral margins of the rays are without angles, being longitudinally 

 straight, or slightly rounded toward the transverse sutures. 



There is a peculiarity in the mode of attachment of the plates or structures above the 

 level of the interbrachials, which induces a strong desire to see the perisomic plates them- 

 selves, which are not preserved in any of the specimens. In Taxocrhuis and Onychocrimis 

 the distal margins of the interbrachials are rounded and smooth, except for slight traces of 

 the attachment of perisome marked by a faint line at the ventral edge of the plates. This 

 line in Taxocrinus is shown in figure 6b of Plate LVII, and the actual margin of the perisome 

 of Onychocrimis in figures 4 and 6 of Plate LXVII. From these it will be seen that there 

 is a considerable space, equal to about the thickness of the plates, between the outer or dorsal 

 surface^and the perisome ; or, in other words, that the perisome is attached at the inner, or 

 ventral, margin of the interbrachials and lower brachials, and not at their dorsal margin. 



In Forbesiocrinus nobilis, however, the surface for attachment of perisome begins at 

 the dorsal margin of these plates and extends inward for their full thickness ; the distal face 

 of the interbrachials and the lateral face of the lower free brachials is but little rounded, and 

 is covered for its entire depth with strong corrugated ridges, or wrinkles, in a dorso-ventral 

 direction. This is perfectly shown by Plate XXIII, figures la, c, d, where the lateral margin 

 of the ray is exposed to the third order of brachials, and in still greater detail by the various 

 figures showing these faces upon isolated plates on Plate XXIV, figures 11, 12, 14, 16. The 

 corrugated margin gradually diminishes in thickness until beyond the second bifurcation, 

 where it becomes mingled in the rugosities and longitudinal grooves of the ventral side (see 

 the same figures on PI. XXIII, and PL XXIV, figs. 17, 20, 21, 22). The structure here 

 described may be compared with that of the later forms of the genus in the detailed diagrams 

 on Plate XXI, figures 1, 2, 3. 



Now this is an entirely different marginal structure from that prevailing in the calyx 

 below the distal face of the interbrachials ; it usually begins at about the middle of IBr.,, 

 below which the brachials are connected with the interbrachials, and the latter with one 

 another, by angular faces and straight sutures, with ligaments lodged in deep fossae 

 (PI. XXIV, figs. 2b, 3, 4, 5a, "jc, 8c, gb, 10c). In the last figure, 10c, the change can be seen 

 to occur on the margin of the same plate. Thus there was at the upper line of the inter- 

 brachials an abrupt and complete change in the mode of union, by which there was attached 

 to the interbrachials, and to the inner margins of the rays for some distance up, some kind 

 of a structure which did not consist of solid or suturally joined plates. At first I took it for 

 granted that these were sutural margins for regular interbrachial plates which had dropped 

 out ; but the fact that in well preserved specimens no trace of any such plates could be found 



