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centrids, judging from Pholidechinus brauni, of which an excellently 
preserved lantern is figured by Jackson (PI. 27). Otheiwise the 
lantern of Echinothurids is of a primitive type, recalling that of 
the palæozoic forms. A perignathic girdle does not appear to have 
existed at all in the palæozoic Echinoids; in the Echinothurids it 
is well developed, though in certain regards peculiar. Is there in 
these facts sufficient reason for separating the Echinothurids sharply 
from the Lepidocentrids? I cannot agree to that. The perignathic 
girdle must have originated once, and the same thing holds good 
for the styloid processes and the pits in the pyramids. Against all 
the otfeer important facts speaking in favour of a genetic relation 
between these two families, these pits and styloid processes appeal 
to me quite trivial and of very slight importance. Likewise the 
faet that the ambulacral plates are compound in the Echinothurids, 
simple in the Lepidocentrids cannot, of course, be taken as an 
argument against the derivation of the former from the latter, 
it is, indeed, only what should be expected. 
There is only one real difficulty for the derivation of the 
Echinothurids from the Lepidocentrids, viz. the faet that the Echi¬ 
nothurids are not known below the Jurassic, while the Lepidocen¬ 
trids are not known beyond the Lower Carboniferous. From the 
Carboniferous proper, the Permian and Trias no connecting form 
is known, and, as Jackson justly emphasizes in a letter to me, 
this is a large geological gap. It is, of course, not very satis- 
factory to have recourse to the imperfeetness of the geological 
record (— as stated above, Jackson must do so himself to 
explain the conspicuous absence of biseriate forms older than the 
pluriseriate, which according to his opinion must have existed ). 
Still it cannot be avoided. I would point to the faet that upon 
the whole the Echinothurids are exceedingly rare fossils even in 
those periods where they are known to have existed. I urther I 
would recall the faet that the oldest known Echinothurid, Pelan- 
echinus corallinus Groom, is in some way the most specialized of 
all Echinothurids, as it has polyporous ambulacra. It is certainly 
