140 
SILUKIA. 
[Chap. VII. 
found in the lower bone-bed. On the other hand, the following species 
here found are unknown in any inferior stratum, namely : — Cephalaspis ? 
ornatus, Egerton; Auchenaspis Salterif, Eg.; Pterygotus Ludensis, Salter; 
Eurypterus pygmseus, Salter. 
Fossils (22). Crustacean and Fishes from the Passage-beds, Ludlow. 
1. PterygotusLu- 
densis, Salter ; 
the caudal por- 
tion, much re- 
duced in size. 
2. Auchenaspis 
Salteri, Eger- 
ton. Head- 
shield. 
3. Cephalaspis ? 
ornatus, Eger- 
ton. (An im- 
perfect head- 
shield; half the 
natural size.) 
3*. Portion of 
fig. 3, magni- 
Besides the portion here figured of the gigantic Crustacean Pterygotus 
Ludensis, its pincers and other parts have been collected at this spot. Mr. 
Salter concludes that this giant of the Crustacea measured not less than 
seven or eight feet in length, and had a proportionate breadth and thick- 
ness. Some further details of its structure will be given in Chapter X. 
On the right bank of the Eiver Teme, opposite Ludlow, a clearer ascend- 
ing succession of strata has been traced, notwithstanding a heavy cover of 
drift and gravel, and chiefly by observing the strata which appear when 
the river is very low. Thus the rocks containing the original Bone-bed 
are observed to slope gently down to the south-east, and to pass under 
light-coloured flagstones, which at the Corn-mill near Ludford are overlain 
by micaceous brownish sandstone. These, again, are followed by reddish 
marls with greenish minute concretions. Whether the same Fish-bed which 
is observable at the Eailroad may not be detected at this point is uncertain, 
as the series is irregularly traceable only$. But above all these strata, 
both grey and red, there occurs a band of light-grey micaceous sandstone 
or grit with carbonaceous markings and spore-cases of cryptogamic Plants, 
and in it Mr. Lightbody has detected remains of Fishes and other fossils. 
Among these we again find the Plectrodus mirabilis, Onchus Murchisoni, 
and Lingula cornea of the lower Bone-bed, though the Platyschisma heli- 
cites of that horizon is no longer detected. It may therefore be naturally 
suggested that this band constitutes the last link in the chain of Silurian 
life. 
t Auchenaspis differs from Cephalaspis in hav- Old Red Sandstone of Scotland and Herefordshire, 
ing the buckler truncated behind, and the nape of I It is manifest that faults must abound near 
the neck covered by two wide square plates with the apex of the elevated Ludlow promontory, and 
ranular surfaces — a character not found in the that dislocations similar to that which occurs at 
road lunate heads of Fishes so common in the the Eailroad may be frequent. 
