366 ROBERT TRACY JACKSON ON ECHINI. 



scattered isolated plates appear, the Oligoporus stage; then the isolated plates become suffi- 

 ciently frequent to form a column when the species character in this respect is attained. 



The interambulacra have five columns of plates at the mid-zone in areas A and C, the two 

 which are most nearly complete. In area A, in addition, a sixth column is represented dorsally 

 by a single plate, dorsal to which the test is wanting (Plate 52, fig. 1). In this area the first two 

 rows are restored as indicated by dotted lines. The fourth column originates in the third row, 

 the fifth column in the eighth row, and the sixth column in the sixteenth row, with a pentagon 

 and ventrall}' situated heptagon as usual. Area C is only partially preserved. The fifth 

 column comes in at about the same zone as in area A. In column 4 there is an aberrant penta- 

 gon X, and, to compensate for its one side short, an adjacent adradial plate has taken on an 

 additional side. Interambulacral plates bear small secondary tubercles, with small spines. 



Lower St. Louis Limestone, Lower Carboniferous, St. Louis, Missouri. The type speci- 

 men is in Mr. Frank Springer's collection 8,118, and was from Professor Hambach's collec- 

 tion. Miss Klem also gives Hardin County, Kentucky, as a locality for the species. 



*Melonechinus springer! sp. nov. 

 Text-figs. 19, p. 59; 237, p. 2.31; Plate .51, fig. 5; Plate 52, figs. 2-6. 



The test is small, high, and spheroidal, with strongly developed melon-like ribs in ambula- 

 cral and interambulacral areas. Known from a single specimen in Mr. Springer's collection. 

 Height 28 mm., diameter through the mid-zone about 32 mm. The actual measurements in 

 diameter are: through B, G, 30 mm. ; C, H, 32 mm. ; and J, E, 35 mm. Width of an ambula- 

 crum at the mid-zone 8 mm. ; width of an interambulacrum 1 1 mm. 



•Ambulacra are wide with, at the mid-zone, six columns of plates in each area, composed 

 of wide occluded, narrow demi-, and one irregular column of isolated plates in the middle line 

 of each half-area (Plate 52, fig. 2). The occluded plates are medially elevated in a high, arching, 

 melon-like ridge. Ventrally in the ambulacra, as developing stages, there are two columns in 

 each half-area (Plate 52, fig. 3) of wide occluded, and narrow demi-plates; higher up scattered 

 isolated plates appear, which, higher again, become more frequent and fall into a vertical 

 columnar series, the species character. Dorsally, near the ocular, as localized stages (Plate 52, 

 fig. 2), there is a single column of ambulacral plates in each half-area. Passing ventrally, 

 next we find two columns in each half-area, then two columns with, in addition, scattered 

 isolated plates, which soon become so frequent as to make a continuous column, the species 

 character. It is interesting to see in this primitive species of the genus how complete the 

 ventral developing and dorsal localized stages are, and how closely they can be compared with 

 the lower genera in the family (text-fig. 237, p. 231). The dorsal localized stages are more 

 complete in this than in some of the higher species of the genus, in which some stages are more 

 or less obliterated by acceleration of development. 



