388 ROBERT TRACY JACKSON OX ECHINI. 



hexagonal columns before this adradial was reached (Keeping assumed that there were eight 

 or nine columns, but this is doubtful). The adambulacral plates show a beveled edge on the 

 adradial suture, and numerous tubercles and spines are present on ambulacral and interambula- 

 cral plates (Plate 58, fig. 3). 



The third specimen in the Museum of Practical Geology Collection 16,303, has dissociated 

 interambulacral plates and a portion of an ambulacral area seen from within, showing well 

 the central position of pores on the interior of isolated plates (Plate 59, fig. 8). 



The specimens in the Sedgwick Museum Collection, Cambridge, England, are from the 

 Aitken Collection, and all consist of small slabs with dissociated interambulacral plates and 

 parts of ambulacra seen from both the exterior and the interior. The interambulacral plates 

 are large, as in the type, and one measured is 8.2 mm. wide by 6 mm. high. 



The British Museum has two specimens from Frome, as above listed. They were pre- 

 sented by the Rev. Canon Jackson in 1877. The larger of these two shows a fragment of an 

 ambulacral area and interambulacral plates with tubercles and spines. An adradial plate shows 

 well the beveled adradial face (Plate 59, figs. 9-11). The British Museum specimen from 

 Thornton-in-Craven is small, but shows a very clear portion of an ambulacrum with a few inter- 

 ambulacral plates. There are onlj^ four columns of plates in a half-area, and the specimen 

 doubtless came from a ventral or dorsal developing area where the full number of columns of 

 plates characteristic of the species does not exist. 



Mr. Hawkins's specimen from near Clitheroe is fragmentarj^, but has an exceptionally 

 good ambulacrum. There are six columns of plates in a half-area, which measures 11 mm. in 

 width. 



*Melonechinus vanderbilti sp. nov. 



Plate 58, fig. 5; Plate 61, figs. 1-4. 



Test large, spheroidal, with strongly marked melon-like ribs. The type and onlj- known 

 specimen is silicified and somewhat distorted; it represents largelj- a mold of the interior, but 

 part of the plates are in place as siliceous pseudomorphs. The great thickness of the plates 

 is well seen in the photographic figure (Plate 58, fig. 5). Height about 93 mm. and diameter 

 through the mid-zone about 124 mm.; this latter measurement is somewhat exaggerated by 

 lateral compression. Width of an ambulacrum at the mid-zone about 38 mm., width of an 

 interambulacrum about 43 mm. 



The ambulacra at the mid-zone have twelve columns of plates composed of wide occluded, 

 narrow demi-, and four irregular columns of isolated plates in each half-area (Plate Gl, fig. 1). 

 The pore-pairs are in peripodia and lie near the outer border of each ambulacral plate. These 

 plates bear numerous secondary tubercles like those of the interambulacra (Plate 61, figs. 1, 3). 

 On the interior, as seen from internal molds, ambulacral plates opposite horizontal sutures are 



