ECHINODERMA TA-^ECHINOIDEA . 583 



making fifth and sixth and even seventh and eighth incomplete 

 columns ; interambulacra nine in adult. 

 Keokuk of Illinois, Missouri and Iowa. 



VII. Melonites Norwood and Owen. 



Test very large, elHpsoidal, grooved longitudinally. Ambulacra 

 broad, concave on both sides of a median ridge, with six to fourteen 

 columns of plates, each perforated near its outer border by a pair 

 of pores. Plates slightly imbricated, the median rows the largest. 

 Interambulacra with four to eleven columns of plates diminishing 

 in number toward the poles. The median plates are six-sided, the 

 two rows adjacent to the ambulacra are five-sided, with the edges 

 indented by the zigzag of the ambulacral plates. Edges of plates 

 sometimes oblique, especially when thick. Tubercles very small, 

 numerous. Spines minute, very thin, needle-like. Periproct cir- 

 cular. Genital plates with three to five genital apertures each. 

 Oculars with a single pore. 



The interambulacra enter the peristomial margin as two plates. 

 Passing dorsally new columns are introduced, the first plates of 

 which are five-sided with the most prominent apex pointing ven- 

 trally. A ventrally adjacent plate of one of the bounding columns 

 is characteristically seven-sided. Newly added plates dorsally are 

 more or less rhombic in Outline. This facilitates the orientation 

 of obscure fragments. Mississippic. 



14. M. multiporus Norwood and Owen. (Fig. 1917, a-c.) 



Mississippic. 



With ten ambulacral and seven to eight or nine interambulacral 

 columns of plates. 



The most characteristic fossil of the St. Louis limestone of 

 Missouri, Indiana, Tennessee, Kentucky, etc. 



VIII. Lepidesthes Meek and Worthen. 

 Large; plates of corona regular and imbricated; interambulacra 

 from 3-7 columns, imbricating aborally and laterally; ambulacra 

 broad, from 8-18 (or perhaps 20?) columns, imbricating adorally 

 with pores in center of plates. Mississippic. 



15. L. wortheni Jackson. (Fig. 1917, J.) Mississippic. 

 With seven to eight columns of ambulacra in adult and interam- 



