MELONECHINUS. 371 



this is in part due to the fact that ambulacral plates bevel over the adambulacrals on the 

 adradial suture so that ambulacra on the interior are necessarily narrower than on the exterior. 

 There are eight columns of plates at the mid-zone in the ambulacral areas, but only two plates 

 dorsally where the ambulacra are in contact with the oculars. In the interambulacra there 

 are six columns of plates at the mid-zone in the areas as far as preserved, but in four inter- 

 ambulacra a seventh column appears dorsally. In the ambulacrum, the demi-plates opposite 

 liorizontal interambulacral sutures are high and laterally fan-shaped, as in Plate 56, fig. 5, also 

 as in that figure, the ]iore-pairs are on the outer portion of occluded plates, the inner portion 

 of demi-plates, and in the middle of isolated plates. The impression of the apical disc is very 

 clearly preserved and measures 11.5 mm. in diameter, which is proportionately 23 % of the diam- 

 eter of the test. This is proportionately larger than apical discs of other specimens as measured 

 from the exterior of the test; but this is in part due to the fact that genital plates probably 

 present a bevel on their adoral face as in Plate 50, fig. 6, so that a measurement taken from the 

 interior would naturally be somewhat larger than one taken from the exterior. The peri- 

 proctal area measures 5.5 mm. in diameter. Impressions of all the oculars and genitals are 

 in place. The oculars all meet the periproct and adorally cover the ambulacra and laterally 

 the interambulacra in part on either side. Melon-like ribs are quite pronounced in this internal 

 mold both in ambulacral and interambulacral areas. This character differs from internal 

 molds in other species in the genus and family that I have seen. The melon-ribs are formed 

 mainly by the thickening of the plates and internal views usually show little or nothing of this 

 character, as seen in Plate 38, fig. 9, and Plate 54, figs. 2, 3 (p. 359). 



The holotype is from the St. Louis Group, Lower Carboniferous, Greenville, Harrison 

 County, Indiana; it is in the Chicago University Collection 6,622, whence it was kindly loaned 

 me by Professor Stuart Weller. Same horizon and locality, F. Braun Collection. Lower 

 Carboniferous, Allen County, Kentucky, Museum of Comparative Zoology Collection 3,147; 

 Lower Carboniferous, Berea, Kentucky, Museum of Comparative Zoology Collection 3,081; 

 Lower Carboniferous, Jackson County, Missouri, Museum of Comparative Zoology Collection 

 3,148 (all from R. T. J. Coll.). 



*Melonechinus stewartii (Safford). 

 Plate 53, figs. 6-8; Plate 'A, fig. 1; Plate .56, fig. 1. 



Mdonites stewartii Safford, 1869, p. 346, Plate 6 (I), figs. la-Id. 

 Melonites muUipora (pars) Keyes, 1895, p. 181. 

 Melonitcs riudtiponis (pars) Klem, 1904, p. 42. 



This specimen is represented by a very perfect external calcareous mold of the dorsal half 

 of the test from about the mid-zone up. So perfect is it that not only outlines of plates, but 

 spines, tubercles, and peripodia can be clearly made out. It is kn(nvn only from Safford's 



