146 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Summary of CHiExopoDA 



Annulata with few or many bristles attached to the sides of 

 the body (whence the name from Greek chaite, bristle, + pous 

 {pod), foot). This class includes (a) the earthworms and their 

 allies, unknown as fossils, {b) Nereis, Prioniodus, and other car- 

 nivorous allies and (c) the vegetable-feeding tube builders. 



The tube-building worms inhabit a tube of their own manu- 

 facture ; these tubes may be formed of lime carbonate, of agglu- 

 tinated particles of sand, etc., or they may be membranous or 

 leathery. These animals have short parapodia which are never 

 used for swimming, and they are devoid of jaws. They include 

 Serpula and Spirorbis. The Chaetopoda are known from the 

 Cambrian to the present. 



Piioniodus (Fig. 56). Ordovician-Devonian. 



A jaw of this form, the only portion found fossil, consists of 



a narrow basal part which supports many, usually five to twenty, 



small teeth, besides a long 

 tooth situated anywhere from 

 the middle to the end of the 

 basal portion. This long tooth 

 is always continued below the 

 basal part. These jaws prob- 

 ably corresponded in position 

 and function to the jaws of 

 Nereis. The name, from Greek 

 prion, a saw, + odous, a tooth, 

 refers to the saw-like arrange- 

 ment of the teeth. 



Very many such cone-shaped 

 teeth, found in the Paleozoic 

 from the Cambrian to the 



Pennsylvanian inclusive, have received the general name of 



conodonts. Some of them may be fish teeth. 



Fig. 56. — A conodont, an imperfect 

 jaw probably of an annelid worm, 

 Prioniodus, from the Genesee (De- 

 vonian) of New York. The projec- 

 tions were pointed ; three of these 

 have been restored. Enlarged; true 

 size indicated by line below. 



