374 ROBERT TRACY JACKSON ON ECHINI. 



The circumference at the ambitus is 130 mm., width of the ambulacra at the mid-zone 18 mm., 

 width of the interambulacra 9 mm. 



The ambulacra are remarkable for the genus in being about twice the width of the inter- 

 ambulacra. At the mid-zone there are ten columns of ambulacral plates in an area, composed 

 of wide occluded, narrow demi-, and three irregular columns of isolated plates in each half- 

 area (Plate 53, fig. 9). As the plates are an internal view, the pore-pairs lie in the inner border 

 of the demi-plates and nea"r the middle of isolated plates; also demi-plates opposite horizontal 

 ambulacral sutures are higher and laterally fan-shaped (compare Plate 56, fig. 5). 



The interambulacra at the mid-zone and throughout the areas, as far as they could be 

 ascertained, consist of three columns of plates. This is the only species in the family that has 

 so few columns in an area, and the only other Palaeozoic Echini with three interambulacral 

 columns in an area are Lepidesthes wortheni Jackson and Meekechinus elegans sp. nov. (Plate 

 67, fig. 8; Plate 76, fig. 1). The three columns continue to the apical disc and laterally are in 

 contact with the oculars as usual (Plate 53, fig. 10), (p. 444). 



The impress of the apical disc is in place dorsally (Plate 53, fig. 10). It measures 10.5 mm. 

 in diameter, which proportionately is 25 % of the diameter of the test. This is relatively 

 the largest apical disc seen in the genus, and is exceptionally large for the Palaeozoic (pp. 87, 

 104). It is to be remembered, however, that the specimen is an internal mold, and on the 

 exterior the diameter of the apical disc would doubtless be smaller (p. 362, 371). The oculars 

 all meet the periproct and ventrally cover the ambulacra and laterally the interambulacra 

 in part on either side. The genitals are high and wide, as usual in the genus, but the pores 

 could not be ascertained. 



This species has the same number of columns of ambulacral plates as the two following, 

 but differs from them radically in the number of columns of interambulacral plates and in the 

 form of the test. It is striking that this species with so highly evolved an ambulacral structure 

 should at the same time have the least evolved interambulacra known in the family. 



.Lower Carboniferous, White Creek, Davidson County, Tennessee; holotype in Vanderbilt 

 University Collection, from which it was kindly loaned me by Professor L. C. Glenn. 



*Melonechinus liratus sp. nov. 

 Plate 53, fig. 11 ; Plate 54, fig. 4. 



Test high and spheroidal, with strongly and sharply elevated melon-like ribs in ambulacral 

 and interambulacral areas. This species is known only from the holotype, which was kindly 

 loaned me for study by Professor L. C. Glenn. Height about 91 mm., diameter about 116 mm., 

 width of the ambulacra at the mid-zone about 36 mm., width of the interambulacra at the 

 same plane 40 mm. 



Ambulacra are nearly as wide as the interambulacra at the mid-zone with ten columns 



