OF SOUTHERN INDIA. 
43 
There can be no difficulty in distinguisliing the Diadematid^. from the Cidarib.e 
hy the incisions on the apertural margin alone, hut the Eciuniem only differ by a 
more irregular position of the pores, though their usual more spheroidal shape, and 
great number of comparatively small primary tubercles, generally admit of recognis- 
ing the members of that family with tolerable accuracy. 
Cotteau gives short characters of thirty-one genera, to which a few fossil and 
recent ones, suggested by Woodward, Agassiz, and others, have to be added. The 
same author sub-divides the family into four groups according to the character of 
the tubercles : — 
1. With the tubercles perforated and cronulated, type Uemicidaris. 
2. „ „ „ „ and non-crenu!atcd, „ Cidaropsii. 
.3. „ „ „ imperforated and crenulated, „ Cyphosoma. 
4. „ „ „ „ and non-crenulated, „ Goniopyyus. 
Out of the thirty-five genera known, two arc represented in the South Indian 
cretaceous dejiosits, namely, Fseudodiadmia, belonging to tlie first, and Orthopsis, 
to the second group. 
The distribution of the Dtadematid/e in time extends from the lowest secondary 
deposits into the present epoch, attaining its greatest development in the cretaceous 
period, both as regards generic as well as specific forms. 
XIV. PSEUDODIADEMA, Besor, 1856. 
The test is always more or less depressed, round or pentagonal, the ambulacral 
areas are conspicuously narrowed towards the apex, and the poriferous zones widened 
and multigeminal on the upper side. Tlie primary tubercles are neaidy equal on 
the two areas and in from two to five or six vertical series, with secondary tuber- 
cles interspersed ; they are perforated and crenulated on the upper margins of the 
bosses. The apex is usually of larger size, and the plate composing it of tender 
structure ; the consequence being that it is very often lost in fossil specimens, ex- 
posing a large pentagonal opening. The apertural margin is more or less distinctly 
decagonal. The spines are cylindrical or slightly compressed, or aciculate, nearly 
smooth, but usually a fine longitudinal striation is observable with the lens. 
The Bseudodiademata are distinguished from other allied genera, such as Ilemi- 
cidaris and Acrocldaris, by the multigeminal pores near the apex fDiplopodia, 
M’Coy, Dcsor), largely developed apical apparatus in connection with the depressed 
shape of the test, and also by the very slight difference in the size of the ambulacral 
and interambulacral tubercles, which are of nearly equal size in the entire extent of 
the two areas. 
The species of Pseudodiadema are the most abundant of the whole family, and 
occur both in Jurassic and in cretaceous deposits ; they were believed to have dis- 
appeared with the uppermost strata of the latter formation, but some recent species, 
lately discovered in America, are stated to belong probably to this genus. 
( 113 ) 
M 
