FROM THE GREY CHALK. 109 



disc, and the outer primary rows are absent from the three uppermost plates (fig. 3 a) . In a 

 large specimen from the Chalk-Marl of Dorset, b, one and a half inches in diameter, the 

 ambital plates have six and eight rows of large tubercles, and two rows of small secondary 

 tubercles. There are sixteen tubercles in each inner series which alone reach the disc, 

 the second, third, and fourth rows disappear as the plates shorten on the upper surface. 

 The small secondary tubercles, situated near the poriferous zones, form a short series 

 between the peristome and equator; they are scarcely larger than granules, but are, 

 nevertheless, mammillated and perforated, and their presence, position, and development, 

 constitute one of the specific characters of this Urchin. The interambulacral tubercles 

 are nearly identical in size with those of the ambulacral areas (fig. 4). They have narrow 

 areolas, prominent bosses, Avith sharply crenulated summits, and large perforated mamil- 

 lons. The miliary zone is narrow at the sides and infra-margin, with two rows of granules 

 of unequal sizes ; at the upper surface it becomes nude and depressed (fig. 3 a) around the 

 discal opening, a character which appears in excess in the var. suhmdum. A number 

 of granules, of different sizes, form hexagonal circlets around the areolas (fig. 5 a). 



The under surface is convex, and the small mouth-opening occupies a slight depres- 

 sion ; the peristome is circular, and its margin notched with feeble entailles (fig. 3 h). The 

 opening for the apical disc was very large (fig. 3 a), widely pentagonal, and sharply angu- 

 lar, extending into the nude portion of the inter-ambulacra. None of the specimens as yet 

 found contain any of the discal plates. (See likewise PL XVIH, figs. 1 a, b, and fig. 2). 



Authors have recognised three distinct forms of this species, which some have de- 

 scribed as so many separate species, whilst others regard them as varieties of one. 



1st. Var. a, variolar e, identical with Brongniart's type form, is found in the Upper 

 Greensand of Wiltshire and the " Chloritic Marl " of Chard (PL XVIII, fig. 2) and I'Etage 

 Cenomanien of Villers-sur-Mer, Calvados, France, from which localities I have specimens. 

 Its upper surface is more or less depressed, and its outline is circular or subpen- 

 tagonal. The inter-arabulacra have four rows of primary and two rows of small secondary 

 tubercles ; the under surface is convex, and the mouth-opening small. 



2nd. The var. h, suhnudum, has the upper surface remarkably nude, from the ab- 

 sence of granules in the upper part of the miliary zone ; the test is higher, and my 

 specimen from the " Chloritic Marl " of Chard has a thicker structure than var. a. 



3rd. The var. c, Moissyi, is still higher and much larger than var. h ; it has a more 

 tuberculous appearance, and from six to eight rows of tubercles in the inter-ambulacra. I 

 have two specimens before me that agree very well with M. Desor's diagnosis of this 

 form, which he considers a good species, or at all events a large variety of P. suhnu- 

 dum. After a critical study of all these forms, I can find no good structural character for 

 separating them, and therefore consider them as varieties of P. variolare, depending on 

 age or habitat for the differences they exhibit in the size, thickness, and number of tuber- 

 cles in the inter-ambulacral areas. 



Affinities and Differences^ — Pseudodiadema variolare is one of the most perfect types of a 



