n6 HISTORICAL PALAEONTOLOGY. 



which only differs from Orthoceras in the bow-shaped form of 

 the shell ; others belong to Phragmoceras, Lituites, &c. ; and, 

 lastly, we have true Nautili, 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 fish 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. 



CHAPTER X. 

 THE UPPER SILURIAN PERIOD. 



Having now treated of the Lower Silurian period at con- 

 siderable length, it will not be necessary to discuss the succeeding 

 group of the Upper Silurian in the same detail the more so, 

 as with a general change of species the Upper Silurian animals 

 belong for the most part to the same great types as those which 



* 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. 



