114 HISTORICAL PALEONTOLOGY. 



the shell. Thus, some Orthocerata 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 Nautili, with their spiral shells, closely 

 resembling the exjsting 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. 



