Echinoid Tests, Diademoida. 
123 
Astropyga (1885, Journ. Linn. Soc., Zool., XIX, pp. 98 and 110). Duncan seems to 
regard the union in those recent Diadematids as a kind of dovetailing, a ridge or 
process of the one plate fitting into a corresponding groove of its neighbour, 
although he admits that there is also much uniting connective tissue. M. lata, 
however, seems to have had transverse sutures more in agreement with the description 
given by Tornquist of the homologous structures in Archaeocidaris rossica (N. Jahrb. 
f. Mineral., 1906, II, p. 47): «Between the edges of the sutural surfaces is a horizontal 
groove. This does not serve for the reception of anv ridge on the adjacent plate, 
but the surfaces abut edge to edge, and the groove serves for the attachment of 
the connective tissue [stroma] which supports the plates». 
The width of the plates, their pecuiiar curvature, and their strong imbrication 
cannot fail to remind one of the Echinothuridae. A comparison of them with the 
interambulacrals of Pelanechinus is therefore of interest. In that genus, as described 
by Mr. T. Groom (1887, Quart. Journ. Geol. Soc., XLIII, p. 703), the infra-ambital 
interambulacrals resemble those of Diademopsis or Pedina, but the supra-ambital 
interambulacrals retain only the main tubercle-series, the rest of the plate being 
covered with minute tubercles und miliaries; at the same time the plates become 
curved as in M. lata, the concavity in the tubercular and adradial region of the 
plate being on the adoral margin; finally, whereas the lower plates are closely 
united by straight sutural edges, those upper ones were flexibly joined and were 
bevelled so as to produce an adapical imbrication. 
It is not contended that M. lata was closely related to, or was a direct ancestor 
of, either Pelanechinus, or Astropyga, or the Echinothuridae. If in any sense 
ancestral, there must still have intervened a series of multituberculate and probably 
stereosomatous forms. 1 But there is an undoubted anatomical resemblance, indicating 
a phvsiological similarity. 
The Echinothuridae are characteristically abyssal at the present day; their 
Cretaceous representatives lived at any rate in deep and still waters; but Pelan¬ 
echinus is found in the Gorallian of Calne, associated with such a fauna as the 
word «corallian» connotes. Why this flexibility should have appeared in such environ- 
ment is hard to say. Equally difficult is it to understand why one Echinoid from 
Jeruzsälemhegy should manifest similar characters. 
Hemipedina. 
1855. Hemipedina T. Wright, Ann. Mag. Nat. Hist. (2) XVI, p. 95. August. 
Genotype: H. Etheridgei (Wright, sub Pedina). 
Subgenus Diademopsis. 
1855. Diademopsis E. Desor, Synops. Ech. foss., p. 79, December. 
Genotype : D. serialis (Ag. in Leymerie, sub Diadema). 
In the general discussion of Diademina, the history of these divisions and 
names has been given at sufficient length, and the sense in which Diademopsis is 
here to be understood has been expounded (pp. 109—113). 
1 See J. W. Gregory, «On the affinities of the Echinothuridae, &c». Quart Journ. Geol. Soc., 
L1II, P . 112 ; 1897. 
