Echinoid Tests, Diademoida. 
107 
pre-existing «disque central» : at least I knovv of no structure in pre-existing 
Echinoid genera to vvhich such a term has been or could be applied. If it 
has any significance it is as showing the mode of origin of the permanent 
and fully formed sur-anal in the Saleniidae. In itself it can hardly be the fore- 
runner of the Saleniid sur-anal; at least Mr. Lambert for one could not, in 
fact does not, admit Palaeopedina as an ancestor of any member of that 
Family. In Pedina , which seems to be the only possible descendant worth 
considering, there is no trace of a sur-anal or any similar modification of the 
apical System. Therefore the change initiated in Palaeopedina globulus seems 
to have led nowhere. 
If it vvere possible to point to certain species as differing obviously from 
the other early Diademina in the elongation of the periproct and development 
of a suranal, and at the same time as forming a homogeneous group, one 
would gladly accept them as a genus. Only one ought, I think, to demand 
something a little more definite than the evidence of «Diadema» globulus. 
But if one were to admit the structure shown in Mr. Lambert’s figures 4, 5, 
and 6 as sufficient warrant for a new genus, it would be necessary to point 
out that the holotype of Hemipedina Etheridgei has a precisely similar peri¬ 
proct, and is therefore equally deserving of independent generic rank. But, since 
that species is the genotype of Hemipedina, this would leave a number of species 
with circular periprocts, but otherwise similar, unprovided with a generic name. 
I do not propose to take any such action with regard to Hemipedina ; and 
for the same reasons I am disinclined to accept Palaeopedina. 
Orthopsis Cotteau (Pal. Franc. Cret. Ech. p. 374, July 1863, and p. 550, June 1864) 
is a perplexing genus. Lambert (1900, p. 29) says that it originally comprised 
four species, of which only two agreed with the diagnosis, so that the type is 
evidentlv Cidarites miliaris d’ARCiiiAC. The four species were Diadema Repel- 
lini A. Gras, D. granuläre Ag. & Desor, Cidarites miliaris d’ARCHiAc, and 
Pseudodiadema ovatum Coouand. Presumably Lambert intends to regard O. 
granularis as a synonym of the earlier O. miliaris, thus following Cotteau, 
Peron, and Gauthier (1876, Ech. foss. Algerie, Terr. secondaires, I, p. 213); 
otherwise the genotype would naturally be O. granularis. This however makes 
only three original species, and it is not clear which of them are the two 
referred to by Lambert ; probably he meant to include 0. ovata. As Lambert 
has well said, Orthopsis, at least as usually diagnosed, differs from Diadem- 
opsis only in the more central position of its main interambulacral tubercles, 
and the straighter line of its pores. These characters, as well as the less 
intimate association of the primary ambulacrals into triads, seem so primitive 
that one is surprised to find them conspicuous in Upper Cretaceous species, 
but recognised in no species older than Bathonian. If Orthopsis is to stand, 
these characters must be regarded as cases of either arrested or retarded 
development. If merely arrested, one should find ancestors with the same 
characters back at least as far as the Lias If retarded, one should be able 
to prove this by other characters of a more advanced nature. 
Taking the first hypothesis, we may remember that Archaeodiadema 
Thompsoni Greg, showed somewhat similar ambulacrals, but since it differed 
entirely in the tuberculation, it is not one of the ancestors we seek. Not much 
