Echinoid Tests, Diademoida. 
125 
The peristomial border is slightly thickened at its adradial end. Between that 
point and the main tubercular series there is a slight but evident excavation vvithout 
any distinct rim. Seen from the inner surface the border shows no trace of a 
perignathic ridge, until a point about corresponding vvith the main tubercular series, 
and at 0.8 mm. from the peristomial margin, vvhere there arises a small conical 
elevation, with its summit at 1.4 mm. from the peristomial margin and about 0.45 mm. 
above the level of the plate, vvhich is here 0.75 mm. thick. This elevation is probably 
a relic of the interradial process of the perignathic ridge. (Fig. 213). 
The following are measurements in millimetres: 
Total height of series of 5 interambulacrals .... 6.4 
Height of largest interambulacral.1.6 
Width of largest interambulacral.circa 6.5 
Width of its scrobicule.. circa 2.2 
Width of its tubercle.1.3 
Width of its mamelon.0.5 
Distance of Perforation from adradial margin . . . . 3.1 
Width of an adjacent secondary scrobicule.1.1 
Relations of the Species. — The considerable width of the interambula¬ 
crals, namely four times their height, the relative smallness of the main tubercles, 
the increase in size of the secondary tubercles, and their serial arrangement, cause 
one to place this fragment in the Diadematidae rather than the Cidaridae. It could 
not, in fact, be referred to Eocidaris, Microcidaris, or Triadocidaris. The possession 
of two or perhaps three series of tubercles to each interambulacral column shows 
that the species must be referred to Diademopsis rather than to Mesodiadema or 
Hemipedina (s. str.). It is, however, removed from the known species of Diadem¬ 
opsis by its more primitive characters: the denticles, the remains of the interradial 
perignathic process, the poorly developed secondary series of tubercles. This renders 
the species of much interest, for it is just such a form as we might expect to find at 
this horizon, on the hypothesis of the independent evolution of Diademopsis from a 
Cidarid like Triadocidaris. 
Diadematoid ambulacrum, a. 
(Plate IX. figs. 214, 215). 
Material. — Portion of an ambulacrum on matrix, from bed i of Section XI 
at Jeruzsälemhegy. Cassian age. 
Description of the Specimen. — The fragment contains about 24 com- 
plete ambulacrals on each side. The length of the whole fragment is 8 mm., but the 
length occupied by the complete ambulacrals is 6.7 mm., measured along the chord of 
the curve. The greatest width of the ambulacrum is 3.8 mm.; it decreases slightly 
adorally. The perradial tract contains a generallv alternating double series of large, 
mamelonate, doubtfully perforate, tubercles, which decrease in size at both ends of the 
ambulacrum. Between these, and almost surrounding them, are smaller ti’bercles, 
varying in size. Where the larger tubercles decrease in size, these smaller ones are 
more numerous, and sometimes those in line with the larger ones are more prominent 
The number of larger tubercles in a single row is from 10 to 12, so that one such 
tubercle approximately corresponds to two ambulacrals. This arrangement, however 
