236 SMITHSONIAN INSTITUTION 



size, it bears the same relation to the plates above it that the first interbrachial does in the 

 regular areas. Its form, and the mode of union on its lateral and distal faces, are shown by 

 text-figure 27, and on Plate XXIII, figure 3. 



28a 28b 28c 



Figs. 27, 2&a-c 



FORMS OF POSTERIOR EASAL 



In Forbesiocrinus: 27, showing angular distal face, and mode of union. In Taxocrinus: 

 28a, showing rounded socket for anal tube, with curved brachials adjoining at one side; 

 28b, oblique view of same, showing how the plate slopes to a thin, rounded distal margin, 

 without surface for sutural attachment ; 28c, dorso-ventral section of same with base of tube 

 in place. 



In most of the species the posterior basal is angular with unequal distal faces, and sup- 

 ports two anal plates; but in some of the later Keokuk or Warsaw species we find it trun- 

 cate, with another plate interpolated below the two. 



In a typical Taxocrinus, on the other hand, while the posterior basal is a very long plate, 

 only about half of it is visible from the exterior, where the apparent upper margin is 

 indented by a socket for the reception of plates of the anal tube. I formerly supposed this 

 indentation to be actually in the upper margin of the plate, but some fortunate preparations 

 of specimens of this genus and Onychocrinus have demonstrated that this is ordinarily not 

 the case. The plate rises above it and behind the tube plates, sloping toward the interior for 

 a distance often as great as, or greater than, the visible portion. The so-called socket is only 

 the lower part of a shallow, median depression extending over the distal edge of the plate to 

 the interior. The posterior basal thins to a narrow, rounded upper edge which is smooth, 

 and is not articulated to any succeeding plates, but forms a margin for the attachment of the 

 finely plated flexible perisome of the tegmen. The form of the plate and of the tube resting 

 on it, now for the first time ascertained, are shown by the text-figures 28a-c, which are made 

 after careful study of a number of specimens dissected to bring out these parts. Some of 

 these are figured on Plate LVII, figures 4, 5, 6a, 8b, which show exactly the form and rela- 

 tions of both plate and tube in the actual specimens. And in the various figures of Onycho- 

 crinns on Plate LXVII may be seen how the perisome is joined to the basal, the tube, and the 

 adjacent brachials. 



As the specimens are ordinarily found we see only the lower half of the basal, which 

 appears short, curved or indented above. The upper half is concealed by the anal tube, 

 which rests in the depression and socket (PI. LVII, fig. 2). This may lie rarely in the 

 middle, in which case the posterior basal would be symmetric ; or it may lie structurally to the 

 right side ; or it may be displaced by pressure, and thus pushed to one side. In most cases 

 the plate appears unsymmetric, with the right upper corner lower than the left. Its general 

 outline in a front view is elongate, rising high between the radials, with nearly parallel sides 

 above, and curving slightly outward below to meet the abutting basals. The upper face ter- 

 minates in a thin, rounded margin, having no plane surface for the attachment of plates. 

 This margin has a shallow median depression, which follows the sloping exterior surface of 

 the plate to a socket about half way down. The notch, or socket, is a mere support for the 

 anal tube, which rests in the socket and groove, usually to the extent of the first two joints. 

 Whether it rests exactly in the middle or to the left or right of it determines the outline of 



