106 
Triassic Echinoderms of Balcony. 
frame of the periproct.» ln P. minima the suranal is unknown, but the inter- 
calation of the posterior oculars betvveen the genitals indicates an excavation 
of the posterior genital by the backward shifting of the periproct, so that the 
existence of a suranal is probable. In P. Pacomei the apex is unknown, but 
the species is placed in Palaeopedina from its resemblance to the genotype. 
So far, then, the evidence for regarding the suranal as a persistent adult 
character is not very convincing. 
The apex is also unknown in Hemipedina Bonei, but that it was cad- 
ucous and, together with the periproct, extended into the posterior interamb- 
ulacrum further than in any Hemipedina or Diademopsis are facts noted by 
Wright and borne out by the specimens in the British Museum. An independent 
examination of these specimens, without, for the time being, reference to the 
literature, convinced me that this species was closely allied to Pygaster. In 
every feature that can be seen, except in the number of tubercle series, it 
agrees with the diagnosis of that genus. As well as possessing the characters 
described by Wright, it is somewhat convex on the upper surface and con- 
cave at the base; the ambulacral areas are prominent and slightly convex, as 
in Pygaster semisulcatus ; in some specimens incipient tubercles are seen just 
below the ambitus, internal to the main series of ambulacral tubercles; the 
tubercular Ornament, especially on the base, is just as in Pygaster-, the test 
has a distinct posterior slope, and the posterior interambulacrum is depressed 
in the adapical region. It is of course impossible to say deünitely that the 
anus was exocyclic; but it is hard to understand the considerable elongation 
of the apical space and the depression of the posterior interambulacrum on 
any other supposition. The smaller number of tubercular series at the ambitus 
and the less relative width of the interambulacrum are correlated characters 
indicating an earlier stage of either racial or individual development, and the 
specimens of H. Bonei themselves show differences in this respect. Having 
observed these points, I was interested to find that Wright himself had been 
tempted to place the species in Pygaster (Monogr. p. 156). Whether it is the 
young of some known species, such as its associate P. sulcatus, which in 
many respects it resembles, or whether it is an independent species at an 
earlier stage of evolution, is still hard to decide; but if the species is to be 
removed from Hemipedina it would go with Pygaster better than with Palaeo¬ 
pedina. Even if it were not far advanced enough to place in Pygaster, it would 
not belong to Palaeopedina, for the development of the apical System in P. 
globulus is not' in the direction of Pygaster, which has no suranal but an 
encroaching madreporite. 
Setting Pygaster (or Hemipedina) Bonei aside, none would contest the 
importance of Mr. Lambert’s observations on Palaeopedina, although not every- 
one will agree that this is «a form of apex in which the more or less pro- 
longed persistence of the central disc has shifted the periproct to the rear». Is 
it not nearer the truth to say that the passage of the periproct to the rear 
has involved partial resorption of the posterior genital, while leaving a space 
to be filled by the increase in size of a periproctal or anal plate, which comes 
into contact with the growing anterior genitals and so produces angles in the 
front border of the periproct ? The half-formed suranal is not the relic of a 
