MELONECHINUS. 385 



sally as preserved, there are only two plates in a row, one primary plate crossing each half-area 

 (Plate 59, fig. 1). This as a localized stage of young plates, is comparable to the typical adult 

 character of Palaeechinus as shown diagrammatically in text-fig. 237, p. 231. Passing ven- 

 trally, we come directly into a stage of two columns of plates in each half-area, demi- and oc- 

 cluded, like the condition characteristic of Lovenechinus. Then we pass quickly through a series 

 of stages marked in each half-area by consisting of three columns (like M. springeri), then four 

 columns (like M. indianensis) , then five columns (like M. multiporus), finally attaining six 

 columns in a half-area, the species character; all as set forth diagrammatically in text-fig. 237, 

 p. 231. This species has as highly evolved a structure of the ambulacrum as any in the genus, 

 yet its dorsal localized stages yield a complete epitome of the phylogenesis of the type. 



The interambulacrum is well preserved dorsally, but part of the plates are wanting in the 

 adoral portion of the area. As there is no dislocation of parts, it seems, however, that the 

 missing plates can be restored with reasonable assurance of accuracy, as indicated by dotted 

 lines (Plate 59, fig. 1). There are only six columns of interambulacral plates: two columns of 

 pentagonal adambulacral and four median columns of hexagonal plates. Dorsally, the plates are 

 nearly rhombic, as is usual in that zone. The adambulacral columns 1 and 2 drop out dorsally, 

 columns 3 and 4 assuming an adradial position, an indication that the specimen, though small, 

 is fully grown and senescent. Ambulacral and interambulacral plates bear numerous small 

 secondary tubercles (Plate 59, figs. 2, 3), and spines which are about 1.5 mm. in length. The 

 spines are relatively stout and tapering from the enlarged base, as usual in the genus. 



This species is known from only one specimen from the Ijower Carboniferous Limestone 

 of Gledstone, Gisburn, Yorkshire. It was collected by R. H. Tiddeman, and is in the Museum 

 of Practical Geology Collection 6,583. The photograph was taken for me through the kindness 

 of Dr. Kitchin of that museum. The species is named for the late Mr. Walter Keeping who 

 published interesting studies of Palaeozoic Echini. 



*Melonechinus etheridgii (Keeping). 



Plate .58, figs. 3, 4; Plate 59, figs. 5-11. 



Mcloiiitrs rthn-idgii Keeping, 1876, p. .398, text-figs. 1-6; Jackson, 1896, p. 240; Klem, 1904, p. 44. 

 Mclniicchinus rtlicridgri Lambert and Thiery, 1910, p. 120. 



This species is known from a number of specimens which are considered below in detail. 

 The largest specimen, which was figured by Keeping and selected by him as the type, is a some- 

 what angular slab measuring 195 by 180 mm. in its larger dimensions. It is covered with a 

 jumbled mass of interambulacral plates, with some nearly complete fragments of ambulacra. 

 A part of this slab is shown in Plate 58, fig. 4 and Plate 59, fig. 6. A second specimen, which is 

 apparently the one referred to by Keeping (1876, p. 398) as being in the British Museum, 



