300 



ROBERT TRACY JACKSON ON ECHINI. 



next coincidently being narrowed on its outer margin and nearly or quite cut off from contact 

 with the interambulacrum (Plate 28, figs. 5, 6). When quite cut off it becomes thereby occluded. 

 Concurrently with this alternating difference the pore-pairs at the mid-zone are slightly biserial 

 instead of purely uniserial as seen ventrally. The condition is quite comparable to that shown 

 in Maccoya burlingtonensis (Plate 33, figs. 1, 2), and is interesting as a parallel taking on of a 

 similar character in an independent family of Echini. 



The interambulacra in each area have nine or ten columns of plates which imbricate strongly 

 aborally and from the center outward and over the ambulacrals. Each plate, instead of being 

 angular, is so rounded in outline that each bounding edge represents very nearly the segment 

 of a circle; the longest segment lies on the left of the plate in the left half-area and on the right 

 of the plate in the right half-area (Plate 28, fig. 1). This longest segment is formed by the 

 free edge of the plate itself. Three shorter segments of circles of the same arc occur on each 

 plate, formed by the overlapping of neighboring plates on the inner side. The structure is much 

 like that shown in text-fig. 32, p. 75. The middle column that imbricates laterally in two 

 directions presents a long sweeping curve which surrounds the plate excepting for a second 

 short curve on the adoral border where in contact with the next succeeding ventral plate of its 

 series. The nearest thing to which the form of these plates can be compared, perhaps, is the 

 scale of an ordinary teleost fish. In addition, a peculiar feature known in no other sea-urchin, 

 is a very thin, delicate flange which bounds and is limited to the longest segment of the circle 

 on each plate (Plate 28, fig. 9) . This flange therefore exists on the long curves on the left side 

 of the left half-area, and on the long curves on the right-hand side of the right half-area. In the 

 median column the flange borders the long curve, which nearly surrounds the plates of this 

 column. The flange is so thin and delicate that it is usually lost by erosion, but it is found 

 in exceptional cases, or in others the slight depression for its reception may be found, not in 

 the plate to which the flange belongs, but in the next contiguous plates. Separated interambula- 

 cral plates present a strong bevel toward the center and adorally as seen from above. As 

 surface features, ambulacral and interambulacral plates alike have many secondary tubercles 

 which support small acicular spines. There are no tubercles on the flanges described. 



The peristome has ten columns of low, wide plates which alone cover the area. Ocular 

 plates are not known, but low, wide genitals exist (Plate 28, fig. 10), each of two preserved 

 having eight genital pores. A typical Palaeozoic lantern exists as described below. 



This species is named for Mr. Frederick Braun of Brooklyn, New York, an ardent and 

 skilful collector, who helped me greatly with the loan of his choicest material. 



Keokuk Group, Lower Carboniferous, Crawfordsville, Indiana, Museum of Comparative 

 Zoology Collection (five specimens, 3,109-3,114, including the holotype no. 3,109); F. Braun 

 Collection; American Museum of Natural History; British Museum of Natural History Col- 

 lection E 10,678 (a specimen given by me, the original of Plate 27, fig. 3; Plate 28, figs. 4-6); 

 Yale University Museum Collection 321 and 322 (two very complete lanterns). 



