AKTHKOPODA — AEACHNIDS. 
89 
a share in biting the food. The six segments of the hind- 
part carry paired plate-like appendages, as in Enrypterida, 
the first foriiiiug the genital operculiun, the rest beaiing gills 
on their hinder surfaces. JAwalIus then diffeis fiom the 
Enrypterida mainly in the fusion and reduction of the 
abdominal segments. In the very young Limidus, however, 
there are nine such segments, not yet fused, and there are 
among the older fossils of this Order many that show a 
similar or greater approach to the Eurypterid plan. The 
first of these exhibited is the Silurian Neolimulus, with at 
least nine free segments ; then Hemiaspis, in which the last 
three are narrower than the 
others and are followed by 
the telson. Bdinuj’vs from 
the Coal Measures has eight 
abdominal segments, of which 
the last two or three are fused ; 
while in the contemporaneous 
Bup'odps yFrestivichia\ the seg- 
ments are reduced to seven, 
and these are fused. If the 
Coal Measure fossils known 
as Cychis are not larval stages 
of the contemporaneous Xi- 
phosura, one can only say that 
they are just what one would 
expect those larvae to have 
been. The resemblance of all 
these early Xiphosura to trilo- 
bites is also too striking to 
be overlooked. Limulns itself 
first appears in the Trias ; 
several specimens from the Solenhofen stone of Kimmeridgian 
age are shown. 
As the Enrypterida were a.ssuming a fresh-water existence 
before vanishing, the Order Scorpionida was making its 
appearance, being first represented in the Silurian rocks by 
what seems to have been an acpiatic, if not actually a marine 
form. This is 1‘alaeopJwnus (Fig. 43), found in both Scotland 
and Gotland. It consists of the same number of segments, 
arranged in the same way as those of Enrypterida, and 
bearing similar appendages. The first two pairs of these 
have strong pincer-claws, the next four pairs are stout and 
end in a single claw, whereas in later scorpions they are 
Kore-body. 
Mid- and 
Uind-body. 
Telson. 
V 
Fig. 42. — Under surface modern 
Xiphosiire, Liviulus pohjjphe- 
mus, the King-crab. About 
the diameter of the real 
animal. 
Gallery 
VIII. 
Table-case 
23. 
Wall-ease 
13c. 
