Echinoid Tests, Diademoida. 
117 
subsidiary tubercles were enlarged, and so arose the true Diademopsis. Hemipedina 
continued, however, and again gave off a branch with enlarged secondary tubercles 
to form Orthopsis, and another branch to form Phymopedina. Some species of 
Hemipedina also showed a tendency towards a posterior movement of the periproct: 
the earlier of these may be separated as Palaeopedina, which led nowhere ; but 
at least one later form, Hemipedina Bonei, shows how a continuation of this 
change initiated Pygaster. 
Mesodiadenta margaritatum 1 n. sp. 
(Plate VIII. figs. 192—197.) 
Diagnosis. — A Mesodiadema with main interambulacral tubercles well devel- 
oped throughout and having scrobicules pronouncedly confluent, definite, slightly 
sunk, but devoid of scrobicular ring. The width of an interambulacral being taken 
as 100, its height is from 25 to 20, distance from centre of tubercle to adradial 
margin 44 to 41, diameter of boss 15 to 14, diameter of mamelon 7.3 to 5.5. 
[These measurements are necessarily based on plates of the oral surface, and in 
each case the former is the more adoral], Adradial suture denticulate, indicating 3 
ambulacrals to an interambulacrum. 
Material. — (a) Holotype, adoral portion of an interambulacrum from Jeru- 
zsälemhegy (figs. 195 — 197); (b) a similar fragment from Cutting I on the Veszprem- 
Jutas railroad (figs. 192—194). Both are of Raiblian age. 
Description of Holotype. — The a column contains 4 well-developed 
main tubercles, which scarcely increase in size at all away from the peristome, also a 
partly atrophied tubercle close to the peristomial border. The b column contains 5 
well-developed tubercles, similar to those in a. The 10 interambulacral plates corre- 
sponding to these tubercles form a thin, somewhat flattened, piece of test, in which the 
sutures between the plates are distinguishable as slight grooves; the aboral edge of the 
fragment corresponds for the rpost part with the sutures. The wide interradial tract is 
slightly but clearly depressed, the depression increasing away from the peristome; also 
the adradial tracts have a marked, rounded slope towards the adradial margin. 
The line of tubercles appears to approach the adradial margin as it nears the 
peristome, although such exact measurements as can be made show that, as in 
Triadocidaris immunita, the more adoral tubercles are really nearer the centres of 
their plates. (Fig. 195). 
Each main tubercle has a minute much depressed mamelon with straight neck. 
There is scarcely any platform. but the boss Starts at once with a convex slope. 
The scrobicules are depressed, small, and so markedly confluent as to be no longer 
circular. There is no scrobicular ring, but at the sides the scrobicular circle is 
definite, since all the extra-scrobicular surface is densely covered with irregulär 
miliaries, of which about 40 are contained in (2.5 mm.) 2 . (Figs. 196, 197). 
The adradial margin of each plate has a slight convex curve, and on its inner 
surface bears about 3 slight denticles: two facts which suggest that the ambulacrals 
may have tended towards a grouping by threes, though not enough to affect the 
simplicity of the plates or the uniseriality of the pore-pairs. The denticles die out 
before reaching the meridional ridge, which is very slight. (Fig. 197). 
1 Pearled, in allusion to the small, closely set tubercles. 
