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 Coiiodonics of the Cambrian, Silurian, and Devonian 



have been regarded as lingual teeth of extinct Cyclostomes. 



The Cyclicc 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- 



c oo,^ D 7 ,1. J !,■ -a- A strous lampreys 'n coat of 

 Fig. 290. — Poh/gnathus dubium Hmde. f J 



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



vonian. (After Hindc.) , • . r '\ ^i 



lamprey origin even tor Arthro- 

 dires has been suggested. The Cyclicv 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 Drcpaniodus, are simple, straight 

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

 Prioniodns, 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 Hypcroircta, or hagfishes, and 

 the Hyperoariia, 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 Hypcroircta {vnZpoa, pal- 



