MELONECHINUS. 359 



3,144 and 3,145; Edwardsville, Floj'd County, Indiana, Museum of Comparative Zoology Col- 

 lection 3,187; Missouri, Museum of Comparative Zoology Collection 3,146; Washington 

 County, Indiana, F. Braun Collection; Alton, Illinois, Yale University Museum Collection 

 315, and 316; Felton, St. Louis County, and Curryville, Pike County, Missouri (Keyes). 



Melonechinus Meek and Worthen. 



Mclonih's Norwood and Owen, 1846, p. 225; Meek and Worthen, 1860, p. 396; Loven, 1874, p. 41; A. 

 Agassiz, 1874, pp. 647, 648, 649; 1881, pp. 78,80; 1904,. p. 80; Duncan, 1889a, p. 15; Jackson 

 and Jaggar, 1896; Jackson, 1896, p. 240; 1899, p. 131 (non Melonites Lamarck, 1822, Animaux 

 sans Vertebres, vol. 7, p. 615). 



Mcloiirchin)i.i Meek and Worthen, 1860, p. 396; Lambert and Thiery, 1910, p. 120. 



Mclccliiiius Quenstedt, 1875, p. 381. 



The test is spheroidal, or exceptionally {M. obovatus) obovate, typically more or less strongly 

 marked by elevated melon-like ribs in ambulacral and interambulacral areas, but in two Euro- 

 pean species the melon-like ribs are wanting. These ribs are due to the thickening of plates, 

 and in most species are slightly, if at all, recognizable in internal molds (pp. 371, 373). In the 

 ambulacra the ribs are formed mainly by the thickening of the two wide median columns of 

 occluded plates. 



Ambulacra are wide, with two columns of wider occluded plates which extend from the 

 center outward, two columns of narrow demi-plates which extend from the interambulacral sutures 

 inward, and, in addition, at the mid-zone, from one to four irregular columns of isolated plates 

 situated between the demi- and occluded plates in each half-area. The demi-plates bevel over 

 the interambulacrals on the adradial suture, and may be traced ventrally to the simpler condi- 

 tion at the peristomal border (Plate 56, fig. 3), but otherwise, as seen from the exterior, are 

 practically indistinguishable from isolated plates. From this structure it results that in Melon- 

 echinus there are at the mid-zone from six to twelve somewhat irregular columns of plates in each 

 ambulacral area, the two halves of an area being equally balanced. In the lower genera of 

 the family the structure of the ambulacra was essentially alike in all species of each genus, but 

 in Melonechinus there is much specific difference, and, as the ambulacrum is the most important 

 structural and physiological part, I consider those species with the simplest ambulacra as the 

 lower species, and those with more complex ambulacra as progressively higher species. As 

 stated, the melon-like ridges in the ambulacra are formed mainly by the median thickening 

 of the two central columns of occluded plates (Plate 60, fig. 3). The pore-pairs are in two more 

 or less deeply depressed valleys on either side. Each pore-pair is surrounded by a peripodium 

 (Plate 61, fig. 8), which, however, is commonly worn away and the pores lie in the outer border 

 of each ambulacral plate. This is the character as seen from the external or distal side (Plate 

 56, fig. 4). As seen from the proximal, or internal aspect (Plate 56, fig. 5), occluded plates 



