MELONECHINUS. 361 



to the typical condition of Lovcncchinus. Further ventrally, some Mattered i-nlatod plate 

 appear (Plate 52, fig. 2), which, as a stage, repeat the typical character of Oligopoi-n-. Then 

 the isolated plates become more frequent so that a third column is attained, which brings it 

 to the condition typical of the lowest species in the genus. As one p:is-es ventrally toward 

 the mid-zone, the number of columns increases progressively until that characteristic of tin- 

 species in hand is attained (Plate 57, fig. 1; text-fig. 245, p. 382; Plate 59, fig. 1). Thu- d>. 

 dorsal localized stages repeat, and in a measure go farther than ventral youthful stages in 

 recapitulating the features characteristic of lower genera in the family or lower species in tin- 

 genus, as diagrammatically shown in text-fig. 237, p. 231. 



In considering the ambulacrum in the Perischoechinoida, Mr. Agassiz (1874, pp. 648, 649) 

 makes certain statements to which it seems necessary to call attention. He says (p. 648), "In 

 Archaeocidaris (Fig. 2) [this figure is Melonechinus multiporus] there are four plates in the 

 ambulacra; each plate forming vertical rows, much as in Hipponoe and Holopneuste-." 

 Archaeocidaris has two columns of simple plates in each ambulacrum (Plate 9, fig. 6), while 

 Hipponoe [= Tripneustes] and Holopneustes both have two columns of compound plate- in 

 an ambulacral area, each plate being composed of three elements. In treating of the ambula- 

 crum of Melonechinus and other members of the Perischoechinoida with many columns of plates 

 in an ambulacral area, Mr. Agassiz (1874, p. 649) compares these with the plates in Stomo- 

 pneustes, Strongylocentrotus, and Heterocentrotus. I do not think that these are parallel 

 cases because in the Centrechinoida there are two columns of compound ambulacral plates in 

 an ambulacral area and the component elements of these compound plates are entirely distinct 

 from the many columns of simple plates that occur in certain Palaeozoic types. 



The interambulacra in Melonechinus are somewhat wider than the ambulacra or excep- 

 tionally (M. obovatus) are narrower. Typically, they are gently or sharply rounded up from 

 the adradial sutures in lower or higher melon-like ribs (Plates 51, 55, 60), which are formed 

 mainly by the thickening of the plates. In two European species the melon-like ribs are appar- 

 ently absent in the interambulacra as they certainly are in the ambulacra. There are from 

 three to eleven columns of plates in an interambulacral area at or above the mid-zone, three 

 (obovatus) being the smallest," and eleven (giganteus) the greatest number known in the genus 

 and the family. Plates of adradial columns are pentagonal and bevel under the ambulacrals 

 as in other genera of the family, and the plates of median columns are hexagonal or the mechani- 

 cal equivalent of the same; that is, if one plate has less than six sides, some other plate has 

 more than six to compensate (p. 68). Dorsally, near the apical disc, young interambulacral 

 plates are more or less rhombic (Plate 57, fig. 1; text-fig. 246, p. 382), but with growth, as they 

 are pushed ventrally by later added plates, the ventral and dorsal borders of each plate are 

 soon truncated, and converted into hexagons, by contact with associated plates in their own 

 vertical series. There is no imbrication of interambulacral plates, the sutures, excepting the 



