460 
ELEMENTS OE GEOLOGY. 
intervening strata full of Upper Ludlow fossils.* At that 
point immediately above the upper fish-bed numerous small 
globular bodies have been found, which were determined by 
I)r. Hooker to be the sporangia of a cryptogamic land-plant, 
]3robably lycopodiaceous. 
Most of the fish have been referred by Agassiz to his pla- 
coid order, some of them to the genus Onchus, to which the 
spine (Fig. 524) and the minute scales (Fig. 525) are sup- 
rig. 525. 
Q 
Shagreen-scales of a placoicl 
fish, Tlielodus parvidens, Ag. 
Bone-bed, Upper Ludlow. 
Fig. 524. 
Onchus tenuistriatus, Agass. Bone-bed. 
Upper Silurian. Ludlow. 
Fig. 526. 
Plectrodus mirabilis, Agass. 
Bone-bed, Upper Ludlow. 
Fig. 52T. 
posed to belong. It has been suggested, however, that On¬ 
chus may be one of those Acanthodian fish referred by Agas¬ 
siz to his Ganoid order, which are so characteristic of the 
base of the Old Red Sandstone in Forfarshire, although the 
species of the Old Red are all difierent from these of the Si¬ 
lurian beds now under consideration. 
The jaw and teeth of another preda¬ 
ceous genus (Fig. 526) have also been 
detected, together with some speci¬ 
mens of JPteraspis Ludensis. As usual 
in bone-beds, the teeth and bones are, 
for the most part, fragmentary and rolled. 
Gray Sandstone and Mudstone^ etc, —The next subdivision 
of the Upper Ludlow consists of gray 
calcareous sandstone, or very commonly 
a micaceous stone, decomposing into soft 
mud, and contains, besides the shells 
mentioned at page 459, Lingula cornea,^ 
Orthis orbicularis,, a round variety of 0, 
elegantula,, Modiolopsis platyphylla^ 
Grammysia cingulata,, all characteristic 
of the Upper Ludlow. The lowest or mud-stone beds 
tain Rhynchonella navicula (Fig. 528), 
which is common to this bed and the 
Lower Ludlow. As usual in Palaeozoic 
strata older than the coal, the brachio- 
podous mollusca greatly outnumber the 
lamellibranchiate (see p. 4^70) ; but the 
latter are by no means unrepresented. 
Among other genera, for example, we observe Avicula and 
* Murchison’s Siluria, p. 140. 
Orthis elegantula. Balm. 
Var. Orbicularis, Sow. 
Upper Ludlow. 
con- 
Fig. 528. 
Rhynchonella navicula, 
Sow. Ludlow Beds. 
