488 The Cyclostomes, or Lampreys 



cases there is doubt as to the real nature of the fossil relic 



in question or as to the proper interpretation of its relationship. 



Thus the Conodontes of the Cambrian, Silurian, and Devonian 



have been regarded as lingual teeth of extinct Cyclostomes. 



The Cycliaz of the Devonian 

 have been considered as minute 

 lampreys, although the vertebral 

 segments are highly specialized, 

 to a degree far beyond the 

 condition seen in the lampreys 

 of to-day. The Ostracophores 

 have been regarded as mon- 



Fio. 290.-Polygnathus dubium Hinde. str US iampreys ; n coat of 



A Conodont from the New York De- mail, and the possibility of a 



vonian. (After Hinde.) . . . . 



lamprey origin even for Arthro- 



dires has been suggested. The Cyclia and Ostracophori were 

 apparently without jaws or limbs, being in this regard like 

 the Cyclostomes, but their ancestry and relationships are wholly 

 problematical. 



The nature of the Conodontes is still uncertain. In form 

 they resemble teeth, but their structure is different from that 

 of the teeth of any fishes, agreeing with that of the teeth of 

 annelid worms. Some have compared them to the armature 

 of Trilobites. Some fifteen nominal genera are described by 

 Pander in Russia, and by Hinde about Lake Erie and Lake 

 Ontario. Some of these, as Drepaniodus, are simple, straight 

 or curved grooved teeth or tooth-like structures; others, as 

 Prioniodus, have numerous smaller teeth or denticles at the 

 base of the larger one. 



Orders of Cyclostomes. The known Cyclostomes are natu- 

 rally divided into two orders, the Hyperotreta, or hagfishes, and 

 the Hyperoartia, or lampreys. These two orders are very dis- 

 tinct from each other. While the two groups agree in the general 

 form of the body, they differ in almost every detail, and there is 

 much pertinence in Lankester's suggestions that each should 

 stand as a separate class. The ancestral forms of each, as well 

 as the intervening types if such ever existed, are left unrecorded 

 in the rocks. 



The Hyperotreta, or Hagfishes. The Hyperotreta (vn^poa, pal- 



