OLDEST KNOWN FOSSIL FISH. 
463 
Orthoceras Ludense (Fig. 535), as well Fig. 533 . 
as the cephalopod last mentioned, oc¬ 
curs in this member of the series. 
A species of Graptolite, G, priodon^ 
Bronn (Fig. 545, p. 467), occurs plenti¬ 
fully in the Lower Ludlow. This fos¬ 
sil, referred, though somewhat doubt¬ 
fully, to a form of hydrozoid or sertu- 
larian polyp, has not yet been met with 
in strata above the Silurian. 
Star-fish, as Sir R. Murchison points 
out, are by no means rare in the Lower 
Ludlow rock. These fossils, of which 
six extinct genera are now knawn in 
the Ludlow series, represented by 18 coswm, stein.) Aymestry; 
species, remind us of various living one-quarter natural size. 
forms now found in our British seas, both of the families As- 
teriadce and Ophiuridce. 
Fig. 534. 
Lituites (Trochoceras) giganteus, J. Sow. 
Near Ludlow*, also in the Aymestry 
and Wenlock Limestones; ^ nat. size. 
Fig. 535. 
Fragment of Orthoceras Ludense^ 
J. Sow. Leintwardine, Shrop¬ 
shire. 
Oldest known Fossil Fish. —^Until 1859 there was no ex¬ 
ample of a fossil fish older than the bone-bed of the Upper 
Ludlow, but in that year a specimen of Pteraspis was found 
at Church Hill, near Leintwardine, in Shropshire, by Mr. J. E. 
Lee of Caerleon, F.G.S., in shale below the Aymestry lime¬ 
stone, associated with fossil shells of the Lower Ludlow for¬ 
mation—shells which differ considerably from those charac¬ 
terizing the Upper Ludlow already described. This dis¬ 
covery is of no small interest as bearing on the theory of 
progressive development, because, according to Professor 
Huxley, the genus Pteraspis is allied to the sturgeon, and 
therefore by no means of low grade in the piscine class. 
It is a fact well worthy of notice that no remains of ver- 
tebrata have yet been met with in any strata older than the 
Lower Ludlow. 
When we reflect on the hundreds of Mollusks, Echino- 
