262 



ICHTHYODORULITES. CONODONTS 



together on the dorsal surface of the head. Head and lateral plates with 

 grooves for the sensory canals. An appendage, covered with plates and 

 supposed to be the pectoral fin, is attached on each side in front. A 

 short dorsal fin, with fulcra in front. Tail covered with scales. Devonian, 

 Pterichthys Ag. (Fig. 142), Asterolepis Eichw., Bothriolepis Eichw. 



Fig. 142. — Pterichthys milleri, restored after Traquair x J. «?) lateral appendages {? pec- 

 toral) ; j joint in appendage ; m supposed upper jaw with notches for narial openingj 

 op operculum ; orb orbits ; double dotted lines indicate grooves for sensory canals. 



Ichthyodorulites. Fin spines consisting of dentin or vasodentin 

 and probably Selachian are indicated by this term. Some of them have 

 been assigned to genera of which nothing is known but these spines. 

 They are fovmd in Palaeozoic rocks. Of such genera may be mentioned 

 Ctenacanthus Ag., Onchus Ag., Homaca.nthus Ag., Psainmoste.us Ag. 



Conodonts are minute denticles met with from the Lower Silurian 

 to the Carboniferous Limestone ; they are sometimes compared with the 

 teeth of lampreys. They consist of a single cusp or of a series of cusps on 

 one base, and appear to be formed of structureless concentric lamellae. 

 They niight be teeth of JMolluscs or Annelids. 



