340 ROBERT TRACY JACKSON ON ECHINI. 



tals are high, wide, and cover the interambulacra in part in each area. They have from three 

 to five genital pores each, situated below the middle in each plate, and arranged in a semi- 

 circle. There is no trace of a madreporite in any specimen seen (Plate 39, figs. 3, 5; Plate 40, 

 fig. 3; Plate 41, fig. 3; Plate 42, fig. 6). In one specimen there are siliceous molds of a few 

 periproctal plates which are small and angular; they were doubtless numerous, as in other 

 species in this family (Plate 42, fig. 6). Jaws are represented only by indistinct impressions in 

 two specimens (Plate 39, fig. 4). 



Burlington Group, Lower Carboniferous, Webb City, Missouri, Museum of Comparative 

 Zoology Collection, the holotype, 3,078, and seventeen other specimens 3,124 to 3,141 (all from 

 R. T. J. Coll.); British Museum Collection E 9,554, E 10,679; Coriscana, Missouri, Yale 

 University Museum Collection, three specimens; Carterville, Joplin, Missouri, Museum of 

 Comparative Zoology 3,189; F. Braun Collection; Keokuk Limestone, Nauvoo, Illinois, Museum 

 of Comparative Zoology Collection 3,197; Burlington, Iowa, F. Springer Collection 8,126. 



I have studied altogether some thirty specimens of this species, a richness of material 

 much exceeding that of most known species of Palaeozoic Echini. The more important features 

 of a few of these will now be considered. 



The Burlington specimen, in Mr. Springer's collection, is a most instructive and important 

 one. It is one of only two specimens so far found outside of Missouri, and with plates all in 

 place (the other specimen being from Nauvoo, Illinois). The Missouri specimens are siliceous 

 molds. The Burlington specimen is excellent for showing surface characters, especially thick- 

 ness of plates, curvature of the ambulacra, and external tubercles (Plate 46, fig. 4). These 

 features in this species are otherwise known only from external molds, as Plate 44, figs. 

 4, 5. There are five columns of interambulacral plates in each area at the mid-zone, 

 and the ventral development of these areas is well shown, there being, as usual in the family, 

 two plates in the basicoronal row, etc. The most interesting feature of this specimen is the 

 perfection of detail shown in the ambulacrum (Plate 42, figs. 1-4), at the mid-zone, and through- 

 out most of the area the typical species character exists as described (Plate 42, figs. 2, 4), with 

 four columns of plates, pore-pairs biserial. Ventrally, near the peristome (Plate 42, fig. 1), 

 the ambulacral plates are primaries with pore-pairs uniserial, like the genus Palaeechinus. 

 Higher up, the plates are alternately primaries, crossing the half-area and enlarged marginally, 

 with occluded plates, which meet the center of the area, but are cut off from the interambulacra 

 by the enlargement of their fellows. Pore-pairs are biserial, which is like the character in Mac- 

 coya. Still higher we find that the primary plates have failed to reach the middle of the half- 

 area and are here transformed suddenly into demi-plates, thus assuming the generic character 

 which is maintained thence to the ambitus. We see that in the development of this type it 

 repeats the character of the lower genera in its family in a very complete degree, as set forth 

 also diagrammaticallj' in text-figure 237, p. 231. Looking at the dorsal region (Plate42, fig. 3), 



