384 ROBERT TRACY JACKSON ON ECHINI. 



In this development of the interambulacrum it is seen that it passes through a series of 

 stages, each of which is. represented by two or more columns of plates. A more primitive 

 type with two columns of interambulacral plates is not known in the family of the Palaeechinidae 

 but may be reasonably hypothecated (p. 220) ; we do know, however, types with three, four, 

 five, six, and seven columns as a character in lower species of the genus, or in lower genera of 

 the family. These, while not perhaps the actual ancestors of the species in hand, may be looked 

 upon as representing phases like those which its ancestors did possess. The earliest added 

 columns come in with great regularity in a definite zone in each area, but in later added columns 

 there is some radial variation in the zone of appearance of columns, this variation progressively 

 increasing in degree up to the eighth column. The radial variation may be still more marked, 

 as in Plate 54, fig. 5, in which there are nine columns of plates in area A, but only eight in the 

 four other areas. Here the radii in one specimen present the limit of variation usually found 

 between different specimens of the species as in Plate 60, figs. 1, 2, where fig. 1 has eight columns 

 of plates in all interambulacral areas and fig. 2 has nine columns in all interambulacral areas. 



*Melonechinus keeping! sp. nov. 

 Text-fig. 237, p. 231; Plate 58, fig. 1; Plate 59, figs. 1-3. 



The test of this species is incomplete but from the fragment that is known it was 

 doubtless spheroidal; melon-like ribs are wanting. This species is known only from the dorsal 

 portion of a corona showing a very good ambulacrum, a fair interambulacrum, and part of a 

 second ambulacrum. It is a light delicate specimen, beautifully preserved in surface detail. 

 This is the second species known from Great Britain ; one species is known from Russia, but all 

 others so far described are from North America. The ambulacrum as far ventrally as pre- 

 served measures about 30 mm. in width, and the interambulacrum at the same zone is esti- 

 mated to be about 33 mm. in width. 



The ambulacrum adorally, at a zone apparently some little distance above the mid-zone, 

 has twelve columns of plates. These consist of rather narrow (for the genus) occluded plates, 

 narrow demi-, and four irregular columns of isolated plates in each half-area. The occluded 

 plates are proportionately much narrower than in the American species of the genus, and as a 

 concurrent feature the melon-like ribs are wanting. The occluded plates, which are beautifully 

 preserved, have the same arc of curvature as the other ambulacral plates in the area. They 

 are only slightly wider than other ambulacral plates, instead of very much wider as in American 

 species of the genus. This same character of relatively narrow occluded plates without 

 melon-like ribs is also a feature of the British species, M . etheridgii, as described. The pore- 

 pairs of keepingi are enclosed by peripodia and lie in the outer portion of each ambulacral plate. 

 About two and one half ambulacral plates equal the height of an adambulacral. As far dor- 



