FLAGIOSTOMl 



103 



The following genera of plagiostomous fishes have been 

 founded on the fossil spines, or " ichthyodorulites," which have 

 been discovered in the " Devonian," or " Old Eed Sandstone 



Fig 23. 

 Centriscus Jmmerosus. 



series." Onchus (represented by 0. semistriatus, 0. heteroyyrus), 

 Dimer acanthus, Haplacanthus, Xarcodes, Nct/idas, Byssacanthus, 

 Cosmacanthus, Homacanthus (fig. 24), Ctenacanthus, Ptyacan- 

 thus, (Jlimatius, Parexus, Odontacanthus, and Pleiiracanthus. 



The genus Homacanthus is founded on small compressed 

 spines, with fine recurved teeth on the back edge, 

 and longitudinal strife on the sides. Specimens 

 of. Homacanthus arcuatus (fig. 24) have been found 

 in Devonian formations near St. Petersburg. 



The carboniferous series of formations includes 

 the mountain limestone, millstone grit, and the 

 coal measures (see fig. 1). In this series the 

 genus Onchus is still represented by the 0. sul- 

 catus, 0. rectus, and 0. subulatus ; and the genus 

 Homacanthus, by H. macrodus and H. microdus, 

 from the carboniferous limestone of Armagh. 

 Pty acanthus, Ctenacanthus, and Pleuracanthus 

 are also forms common to the Devonian and 

 carboniferous periods. The spine of the latter genus is denti- 

 culated along both margins, a structure which is presented, 

 in existing Plagiostomes, only by species of the ray family ; 



Fig. 24. 



Homacanthus 



arcuatus. 



(Devonian, 



Russia.) 



