130 HISTORICAL PALAEONTOLOGY. 
The last and highest group of the Ao//usca—that of the 
Cephalopoda —is still represented only by TZetrabranchiate 
forms; but the abundance and variety of these is almost 
beyond belief. Many hundreds of different species are known, 
chiefly belonging to the straight Orthoceratites, but the slightly- 
curved Cyrtoceras is only little less common. There are also 
numerous forms of the genera Phragmoceras, Ascoceras, Gyro- 
ceras, Lituites, and Vautilus. Here, also, are the first-known 
species of the genus Gomiatites—a group which attains con- 
siderable importance in later deposits, and which is to be 
regarded as the precursor of the Ammonites of the Secondary 
period. 
Finally, we find ourselves for the first time called upon to 
consider the remains of undoubted vertebrate animals, in the 
form of fishes. The oldest of these remains, so far as yet 
known, are found in the Lower Ludlow rocks, and they con- 
sist of the bony head-shields or bucklers 
of certain singular armoured fishes belong- 
ing to the group of the Ganoids, repre- 
sented at the present day by the Stur- 
geons, the Gar-pikes of North America, 
and a few other less familiar forms. The 
principal Upper Silurian genus of these is 
FPteraspis, and the annexed illustration (fig. 
74) will give some idea of the extraordi- 
nary form of the shield covering the head 
in these ancient fishes. ‘The remarkable 
Fig. 74.—Head-shield stratum near the top of the Ludlow for- 
iiian Somes Cane mation known as the ‘ bone-bed” has 
Murchison.) also yielded the remains of shark-like 
fishes. Some of these, for which the name 
of Onchus has been proposed, are in the form of com- 
pressed, slightly-curved spines (fig. 75, A), which would appear 
Fig. 75.—A, Spine of Onchus tenuistriatus ; B, Shagreen-scales of Thelodus. Both 
“front "the ‘bone-bed ” of the Upper Ludlow rocks. (After Murchison. ) 
to be of the nature of the strong defensive spines implanted 
in front of certain of the fins in many living fishes. Besides" 
these, have been found fragments of prickly skin or shagreen 
(Sphagodus), along with minute cushion-shaped bodies (Zheo- 
