230 ROBERT TRACY JACKSON ON ECHINI. 



ambulacral plates at the mid-zone. In this genus ventrally there are four plates in each area 

 instead of two as in the preceding genera, although exceptionally in one known case, as a re- 

 gressive variant, in one area simple primaries occur (Plate 57, fig. 3). In Melonechinus springeri 

 ventrally the plates are demi- and occluded, higher up isolated plates appear; at the mid-zone 

 there are three definite columns in a half-area. Above the mid-zone we pass into an area of 

 demi-, occluded, and scattered isolated plates, then demi- and occluded only, and near the 

 ocular primaries only. We see in this and other species of the genus that dorsally a simpler 

 condition exists than that found at the ventral border of the corona, emphasizing the value of 

 localized stages in studies of development. 



In Melonechinus indianensis ventrally the plates are demi- and occluded, higher up demi- 

 and occluded, with scattered isolated plates; higher again three columns, and at the mid-zone 

 four columns of plates in a half-area, the species character. Passing dorsally we find three col- 

 umns, then two, and near the ocular primaries only. In Melonechinus multiporus the same 

 condition occurs, but a little more complex. At the ventral border plates are demi- and 

 occluded, higher up the same with scattered isolated plates, then three columns, then four, and 

 at the mid-zone five columns in a half-area as the full specific differential. Passing dorsally, 

 we fall into four columns, then three, two, and near the ocular primaries only may occur as 

 in the same area in all lower species and associated genera. The extreme condition of the 

 family occurs in Melonechinus giganteus. Here from conditions of preservation certain stages 

 are wanting, but ventrally there are demi- and occluded plates, next as far as preserved an 

 area with three columns, then one with five, and at the mid-zone there are six columns in the 

 half-area. Melonechinus keepingi at the mid-zone has six columns in the half-area as in 

 giganteus, and the dorsal area of this species was therefore used to complete the diagram, 

 which I could not do in the case of giganteus. Passing dorsally from the mid-zone, we find a 

 condition of five columns, then four, three, two, and finally primary plates only close to the 

 dorsal border of the area. 



TEXT-FIG. 237. Illustrating the structure of the ambulacrum in representatives of the Palaeechinidae as seen at the 

 mid-zone, with the phylogenesis of the same as shown by ventral youthful stages and dorsal localized stages of the species 

 shown. Numerals indicate number of columns of ambulacral plates in a half-area. (Compare text-figs, on pp. 54, 59). 



Palaeechinus elegans M'Coy (from Plate 31, fig. 1). 



Maceoya burlingtonensis (Meek and Worthen) (from Plate 33, figs. 1,2). 



Lovenechinus missouriensis (Jackson) (from Plate 42, figs. \-V). 



Oligoporus danae Meek and Worthen (from Plate 50, figs. 1, 7, 8). 



Melonechinus springeri sp. nov. (from Plate 52, figs. 2, 3). 



Melonechinus indianensis (Miller and Gurley) (from Plate 53, figs. 1-3). 



Melonechinus muUiporus (Norwood and Owen) (from Plate 56, figs. 2, 3, 4, 6; Plate 57, fig. 1, and text-fig. 245, 

 p. 382. 



Melonechinus giganteus (Jackson). Of this species the structure from the ventral border to the mid-zone is repre- 

 sented below the black line (from Plate 59, figs. 12, 13, and Plate 61, fig. 8). 



Melonechinus keepingi sp. nov. at the mid-zone has the same character as giganleus, and its dorsal area is represented 

 above the black line (from Plate 59, fig. 1). 



