TAXOCRINIDAE 413 



this is now reinforced by three other good specimens showing the delicate, rounded anal tube, 

 without any solid connection with the broadly rounded rays adjoining it (figs. II, 12, 14). 



An important thing brought out by the new material is the exact nature and function 

 of interbrachial plates, which serve to fill Up spaces between the brachial elements as required 

 by the growth of the viscera. They begin to form at the suture lines on the inner surface of 

 the calyx wall, and gradually force their way outward like wedges ; thus the exterior surface 

 of those plates which extend through is much less than the interior, and some never reach 

 the outer surface at all. All this is beautifully illustrated by the preparations figured 1 on 

 Plate LXI; figures ic, d show the opposite surfaces of the interbrachials in part of an 

 interradius (numbered 1, 2, at the right lower corner of fig. lb), together with the adjoin- 

 ing brachials ; of the seven plates plainly visible at the inner surface only two and a mere 

 point of a third have succeeded in reaching the exterior, and probably no more would have 

 done so, as this is a very mature specimen, having doubtless reached its full size ; the other 

 four remained simply as wedges, just as the one at the left side of no. 1 appears' in the end 

 view, figure id. The relations of the plates are still better shown by figures 3a, b, c, d, from 

 a detached part of the same specimen, in which the numbers and lettering on the different 

 plates sufficiently explain them, if it is added that the part of the ray at the right is displaced, 

 and stands somewhat lower than it would in its natural order. In the middle interradius 

 6 large plates at the interior are represented by only 4 much smaller ones at the exterior, 

 while those that started and formed good-sized wedges in the second axil did not get 

 through at all ; their mode of growth, and the corresponding opposite sloping of the brach- 

 ials, may be seen in figure 3c 



In this species 4 is about the maximum number of iBr that appear externally, and in 

 younger specimens the number is progressively less, until none whatever are seen (fig. 16). 

 Hence as a specific character this is about the weakest, and only of service if we deal in 

 general averages ; if merely based on definite numbers it is wholly worthless. In this 

 species the peculiarity of the interbrachials is not simply the small number, but that the areas 

 attain almost their full width at once — not increasing much above the first range. 



The preparation also brings out some very peculiar modifications in the shape of the 

 primary brachials due to the proximity of the anal structures. Figures 2a, b show the first 

 5 plates in the left posterior ray ; the radial, which is very short exteriorly (fig. 2a, no. 1 ) , 

 is enormously produced inward at the right margin next to the anal area, and enlarged at 

 the interior until it occupies about half the area of the whole five plates (fig. 2b) ; plates 2 

 and 4 on the other hand are greatly reduced, and 3, a good-sized plate exteriorly, does' not 

 reach the inside but remains a mere wedge, as shown in figure 2d. It is probable that similar 

 distortions of the plates exist in the right posterior ray, as the radial on that side is always 

 much smaller than the radials in the other rays (figs. 11, 12, 14). This feature seems to 

 exist throughout the genus. 



Another peculiarity is that in the posterior rays one of the primibrachs is much smaller 

 than the others, narrowing toward the anal area to a knife edge, and sometimes not reach- 

 ing it at all (fig. 14). This is due to the great curvature of the rays at this point. 



The secundibrachs are inconstant, varying from 3 to 4 in about equal numbers through- 

 out 16 specimens. 



This species has only been found high up in the Upper Burlington limestone, near or 

 in the transition beds toward the Keokuk ; most of the specimens studied were found in a 

 single colony. 



Types. Author's collection. 



Horizon and locality. Lower Carboniferous, upper part of Upper Burlington lime- 

 stone ; Burlington, Iowa. 



