14 



CONNECTICUT GEOL. AND NAT. HIST. SURVEY. 



[Bull. 



The Cambrian system, at the base of the Paleozoic, has thus 

 far failed to yield any indication of the presence of backboned 

 creatures. Neither Chordates nor " Protochordates," that is, 

 primitive forerunners of the vertebrate phylum, make their ap- 

 pearance in the geological time-scale until the Ordovician, after 

 which they continue sparsely throughout the Silurian. From 

 what lower group of organisms the primitive progenitors of the 

 vertebrate stem were descended, and during what period the 

 hiatus between diverse phyla was bridged over, we have no 

 means of knowing. The absence of transitional forms, or indeed 

 even of Protochordates, in strata anterior to the Ordovician 

 is not a very significant fact, when it is considered that the 

 primitive forerunners of chordate animals were probably soft- 

 bodied, and therefore incapable of preservation in the rocks. 

 Although numerous indications of fish-like vertebrates have been 

 obtained at different localities both in this country and abroad, 

 as, for instance, from the Ordovician of Colorado, Montana, and 

 Scotland, it is not until the Silurian that their remains are found 

 satisfactory enough for discussion. The dominant forms of fish 

 life that we are acquainted with from rocks of this age belong to 

 the lowly group of Ostracophores ■ — creatures which differ from 

 Fishes proper to such an extent that they are usually included in 

 a separate class (Agnatha). They have incompletely formed 

 jaws, are destitute of paired fins, and are without calcified en- 

 doskeletal parts (Fig. i). On the other hand, as their name 

 implies, they are protected by a shell-like external covering, 

 whose elaboration can be traced through a number of successive 

 stages. Toward the close of the Devonian they become entirely 

 extinct, without leaving descendants. 



Fig. I. Cephalaspis murchisoni Egerton. Lower Old Red Sand- 

 I^"*"'^ '■^,°c'^*''^- Head-shield seen from above, tail twisted to 



