372 ROBERT TRACY JACKSON ON ECHINI. 



original type specimen which is in the Vanderbilt University Collection, and was kindly loaned 

 me by Professor L. C. Glenn of that institution. 



As gathered from the dorsal half, the test is high and spheroidal, with strongly and sharply 

 elevated melon-like ribs in ambulacral and interambulacral areas. These are shown in the 

 plaster cast of the type (Plate 54, fig. 1). Diameter 77 mm., width of the ambulacra near the 

 mid-zone about 23 mm., width of the interambulacra in the same plane about 24 mm. 



The ambulacra are of about the same width as the interambulacra near the mid-zone, 

 with eight columns of plates, wide occluded, narrow demi-, and two irregular columns of isolated 

 plates in each half-area (Plate 53, fig. 6). The occluded plates medially are elevated in a high, 

 steeply rounded, melon-like curve. Pore-pairs are in peripodia, situated in the outer portion 

 of each ambulacra] plate. The figure of ambulacral detail is drawn directly from the original 

 mold, therefore the orientation is reversed from what it would be if seen from the exterior. 

 The interambulacra are elevated, in high, rounded, melon-like ribs, which laterally dip down 

 steeply to the adradial sutures. In four areas there are seven columns of interambulacral 

 plates at or above the mid-zone, but in area I there are only six columns. In areas E and G 

 the seventh column originates above the lower border, as preserved, with a pentagonal plate 

 bearing a heptagonal plate on the right ventral border (Plate 53, fig. 8). This figure is also 

 drawn directly from the original external mold, therefore the orientation is reversed from what 

 it would be if seen from the exterior. Ambulacral and interambulacral plates bear small 

 secondary tubercles, and spines about 3 mm. long. 



Dorsally, the apical disc is in place, and it measures proportionately about 17 % of the 

 diameter of the test (Plate 56, fig. 1, was drawn from a wax cast of the original mold). There 

 are low, small, imperforate oculars all reaching the periproct, and five wide, high genitals. 

 Genital I has three pores situated as usual toward the ventral border of the plate. In the other 

 genitals the impressions of pores are somewhat scattered and doubtful as to the number of 

 pores in each plate. In this specimen impressions of periproctal plates are in place. They 

 are small, angular, and evidently fill the area, as in Maccoya and Palaeechinus where these 

 plates are better known. In all cases these plates are closely similar to those of a Recent 

 Eucidaris (text-fig. 66, p. 98). This is the only specimen of the genus Melonechinus in which 

 the periproctal plates are known. 



This species is close to muUiporus, but differs from it in that the melon-like ribs are much 

 higher and more sharply rounded (compare Plate 55), and the ambulacral areas are propor- 

 tionately somewhat wider. There are eight columns of ambulacral plates in stewartii whereas 

 there are usually ten columns in muUiporus; also there are seven or six columns of interambula- 

 cral plates in each area in stewartii, whereas in muUiporus there are typically eight or nine 

 columns, very rarely seven, and as few as six are unknown. 



Professor Safford says of this fossil that the precise locality is not known to him, but that 



