114 HISTORICAL PALAJONTOLOGY. 
the shell. Thus, some Ovrthocerata have been discovered 
measuring ten or twelve feet in length, with a diameter of a 
foot at the larger extremity. ‘These colossal dimensions cer- 
tainly make it difficult to imagine that the comparatively small 
body-chamber could have held an animal large enough to move 
a load so ponderous as its own shell. ‘To some, this difficulty 
has appeared so great that they prefer to believe that the 
Orthoceras did not live in its shell at all, but that its shell was 
an internal skeleton similar to what we shall find to exist in 
many of the true Cuttle-fishes. There is something to be said 
in favour of this view, but it would compel us to believe in the 
existence in Lower Silurian times of Cuttle-fishes fully equal 
in size to the giant “Kraken” of fable. It need only be 
added in this connection that the Lower Silurian rocks have 
yielded the remains of many other Tetrabranchiate Cephalo- 
pods besides Orthoceras. Some of these belong to Cyrtoceras, 
which only differs from Orthoceras in the bow-shaped form of 
the shell; others belong to Phragmoceras, Lituites, &c. ; and, 
lastly, we have true /Vaw?z/z, with their spiral shells, closely 
resembling the existing Pearly Nautilus. 
Whilst all the sub-kingdoms of the Invertebrate animals are 
represented in the Lower Silurian rocks, no traces of Verte- 
brate animals have ever been discovered in these ancient 
deposits, unless the so-called ‘‘Conodonts”’ found by Pander 
in vast numbers in strata of this age * in Russia should prove 
to be really of this nature. These problematical bodies are of 
microscopic size, and have the form of minute, conical, tooth- 
shaped spines, with sharp edges, and hollow at the base. 
Their original discoverer regarded them as the horny teeth 
of fishes allied to the Lampreys; but Owen came to the con- 
clusion that they probably belonged to Invertebrates. The 
recent investigation of a vast number of similar but slightly 
larger bodies, of very various forms, in the Carboniferous rocks 
of Ohio, has led Professor Newberry to the conclusion that 
these singular fossils really are, as Pander thought, the teeth of 
Cyclostomatous fishes. ‘The whole of this difficult question 
has thus been reopened, and we may yet have to record the 
first advent of Vertebrate animals in the Lower Silurian. 
* According to Pander, the ‘‘Conodonts” are found not only in the 
Lower Silurian beds, but also in the ‘‘ Ungulite Grit ” (Upper Cambrian), 
as well as in the Devonian and Carboniferous deposits of Russia. Should 
the Conodonts prove to be truly the remains of fishes, we should thus have 
to transfer the first appearance of Vertebrates to, at any rate, as early a 
period as the Upper Cambrian. 
