Echinoid Tests, Diiidernoida. 
109 
was referred to Orthopsis by P. de Loriol (1884, Rec. Zool. Suisse, I, p. 614) 
on the evidence of a specimen from Portugal. The holotype (Brit. Mus. E 1501) 
is poorly preserved, but the alternation of its tubercles was already noted by 
Wright. The peristome is not exposed. In Hemipedina microgramma Wr. 
doubtfully assigned to the Cornbrash, the much worn holotype (Brit. Mus. 
20746 c) shows similar remoteness of main tubercles at the peristome and 
similar alternation of tubercles; here, however, there are two series external 
to the main one, so that, counting from the adradius, the adapically shifted 
tubercles are the 2nd. and 4th. The Bathonian Hemipedina Davidsoni Wr. 
has been referred to Orthopsis by Pomel, Cotteau, and Lambert ; but the 
holotype (Brit. Mus. E 1667) does not show the above-mentioned characters 
at all clearly, so that I should regard it as very little, if at all, modified in 
the direction of a true Orthopsis. If, on the other hand, one turns to the 
Senonian 0. Morgani and O. globosa both of Cottealt & Gauthier (1895, 
Mission Sei. Perse, pp. 87, 89) and the Maestrichtian 0. perlata Noetung 
(1897), one finds similar alternation of interambulacral tubercles, and similar 
relation of tubercles to the primary ambulacrals. In 0. Morgani the pore- 
pairs are said not to multiply around the peristome, but no figure is given. 
The pore-pairs show a deviation from the straight line ; but this does not, as 
Lambert says, bring it nearer Diademopsis, for the deviation is not an arcu- 
ation around the main tubercle as in the ordinary Diadematoid type, but is a 
curve in the opposite direction, and may be regarded as an intensifleation of 
the reversion to straightness, and as due to the increased size of the second¬ 
ary tubercles. 
Without discussing other species, of which I have not examined 
specimens, I feel convinced that there is more to be said for the genus Orth¬ 
opsis than has been said by Lambert or even Cotteau, or than finds a place 
in any diagnosis yet given. It seems a fair conclusion that Orthopsis is a 
post-Bajocian modification of Diademopsis, displaying in its ambulacra a some- 
what deceptive appearance of partial reversion to ancestral structure. At any 
rate — and this is the important corollary on the present occasion — Orth¬ 
opsis is not to be expected from the Triassic rocks. 
Hemipedina and Diademopsis. — The ground is now clear for consideration 
of these two genera which, from the very year of their foundation (1855) 
by Wright and Desor respectively, have greatly troubled systematists. 
Lambert (1900) makes Diademopsis a subgenus of Hemipedina on p. 28, but 
treats it as a genus, with independent diagnosis, on p. 6. Whether as genus or 
subgenus, it can be justified only if it constitutes a genetic group. 
Hemipedina has priority. Wright never distinctly selected a genotype, 
but Lambert (1900, p. 28) has fixed on H Etheridgei, while Savin (1905, 
Bull. Soc. Jsere, ser. 4, VIII, p. 115) selected H. tuberculosa, and might 
have justified his overriding of Lambert ’s choice by pointing to Wright’s 
Statement (Pal. Soc. Monogr. Ool. Ech., p. 166) that H. tnberciilosa «forms 
the best type of that section of the genus, which has two rows of tubercles 
in the inter-ambulacra, with a wide miliary zone». It is, however, clear from 
the word «best» that Wright was not here using «type» in the strict syste- 
