142 GEOLOGICAL DISTRIBUTION. 



absence of positive indications of the existence of vertebrated ani- 

 mals. The only objects that have thus far been described as per- 

 taining, with any show of probability, to the members of this highest 

 division of the animal kingdom, are the singular bodies known as 

 conodonts, which, in the opinion of their discoverer, Pander, and 

 of some other naturalists, represent the teeth of fishes belonging to 

 the order of the myxinoids (hags and lampreys), with the exception 

 of the lancelet (Amphioxus) the lowest of the entire class of Pisces. 

 The weight of opinion, however, seems to relegate these problem- 

 atical bodies to the Invertebrata, and not improbably, as has been 

 urged for some of these forms, they represent the jaw-teeth of cer- 

 tain annelids. 



Silurian Fauna. — The fauna of the Silurian period marks a 

 decided advance upon its. predecessor. The chain of organisms 

 which, with the exception of the somewhat doubtful conodonts, 

 was hitherto constituted exclusively by the members of the inver- 

 tebrate series — sponges, echinoderms, mollusks, articulates — exhib- 

 its here for the first time indisputable representatives of the more 

 highly organised group of the vertebrates ; but not until the Upper 

 Silurian deposits are reached.- We here meet with the remains of 

 two distinct orders of fishes, the sharks or dog-fishes (Elasmo- 

 branchii), as represented by Onchus and Thelodus, and the bucklered 

 Ganoidei — Pteraspis — the former still very abundant in the modern 

 seas; the latter, which include, among other forms, the sturgeon 

 and alligator- gar, probably nearly verging on extinction.* In both 

 these orders the osseous framework or skeleton is frequently in a 

 more or less imperfect condition — complete ossification being the 

 exception rather than the rule — and hence, in so far, these primi- 

 tive vertebrates exemplify a low grade of organisation compared 

 with those — like the bony fishes, and most of the animals above 

 them — in which the vertebral column is completely ossified, or 

 reaches its furthest development. Nor are other characters want- 

 ing proving inferiority of organisation. We have here, therefore, 

 another illustration of the very important fact — a fact sustaining 

 the inference of the progressive evolution of higher from lower 



* The oldest fislies were, until recently, supposed to belong to the British 

 Ludlow beds ; hut the discovery, by Professor Claypole, ot ichthyic fragments 

 in deposits below the " VVatcr-Lime" of Pennsylvania would seem to remove 

 them still farther back in the geological scale. 



