370 ROBERT TRACY JACKSON ON ECHINI. 



exceptional (compare tetragonal plate of ninth column in Plate 50, figs. 2, 3, and Plate 59, 

 fig. 14; pp. 358, 391). To compensate for the two sides wanting there are two heptagonal plates, 

 H, one on its left and one on its right ventral border. The sixth column originates on the left 

 of the center in the tenth row, with a pentagonal plate which has a heptagon on its left ventral 

 border. The seventh column originates in the fourteenth row in the middle of the area with a 

 heptagon on its right ventral border. The eighth column, consisting of only two plates, origi- 

 nates in the eighteenth row. Both plates of this column are pentagons, and a heptagonal plate 

 lies on the ventral border of one and the dorsal border of the second pentagon, compensating 

 for the one side lacking in each. Dorsally, in this specimen (Plate 53, fig. 1) the adambulacral 

 columns 1 and 2 drop out before reaching the apical disc, so that dorsally columns 3 and 4 

 assume an adradial position. In addition, one other column has dropped out in several areas. 

 In area C column 4 drops out so that column 6 assumes an adradial position adapically; in 

 area E, column 5 drops out, and in area G, column 6 drops out, before reaching the apical 

 disc. 



In the type the apical disc is in place and complete; it measures proportionately 16 % 

 of the diameter of the test. The five oculars all reach the periproct, are imperforate, and 

 ventrally cover the ambulacra and laterally the interambulacra in part on either side. The 

 genitals are high and wide, bounded by the periproct dorsally, the oculars laterally, and ven- 

 trally, in a long curving line, by the interambulacra, which laterally extend in a narrow, wedge- 

 like fashion to their points of ocular contact. There are three pores in genital G, in other 

 plates no pores or fewer than three; they have doubtless been obliterated in the process of 

 silicification (p. 363). 



A small specimen in Mr. Braun's collection from Greenville, Indiana (Plate 51, fig. 8) 

 is completely silicified and preserved without distortion. There are eight columns of ambula- 

 cral plates and six columns of interambulacral plates respectively in each area. The angles 

 of the melon-like ribs are exceptionally sharp and well preserved. The apical disc measures 

 proportionately 18 % of the diameter of the test. 



A specimen in the Museum of Comparative Zoology Collection 3,147, from Allen County, 

 Kentucky (Plate 51, fig. 9), is silicified, and stained a 3^ellow amber color; much wear has 

 rounded the outlines. It has the eight columns of ambulacral plates, and in each interambula- 

 crum six columns of plates, the species character. Another specimen in the Museum of Com- 

 parative Zoology Collection 3,081, from the Lower Carboniferous, Berea, Kentucky, is of much 

 interest as a silicified internal mold, and is the best internal mold of any specimen of the genus 

 known to me. The specimen although partially wanting is perfect in form and free from all 

 distortion. It measures 40 mm. in height and 50 mm. in diameter. At the mid-zone the 

 ambulacra measure 12 mm. in width, and the interambulacra 27.5 mm. in width. These 

 proportions differ . considerably from those given for external measurements in the type, but 



