PLAGIOSTOMI. 



123 



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

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

 Pimer acanthus, Haplacanthus, Kar codes, Kaulas, Byssacanthus, 

 Cosmacanthus, Homacanthus (fig. 36), Ctenacanthus, Parexus, 

 and Odontacanthus. 



The genus Homacantluts is founded on small compressed 

 spines, with fine recurved teeth on the back 

 edge, and longitudinal striae on the sides. Spe- 

 cimens of Homacantluts arcuatus (fig. 36) have 

 been found in Devonian formations near St. 

 Petersburgh. 



The carboniferous series of formations includes 

 the so-called slates, mountain limestone, millstone 

 grit, and the coal measures (see fig. 1). In this 

 series the genus Onchus is still represented by the Fig. 36. 

 0. mlcatus, 0. rectus and 0. subulatus ; and the Homacanthus 



arcuatus. 



genus Homacanthus, by H. macrodus and H. micro- (Devonian, 

 das, from the carboniferous limestone of Armagh. Russia 

 Ctenacanthus is common to the Devonian and carboniferous 

 periods. The spine of Pleuracanthus (fig. 37) is denticulated 

 along both margins, a structure which is pre- 

 sented, in existing Plagiostomes, only by species 

 of the ray family ; it belongs to an extinct form, 

 to which the monk-fish of the present day offers, 

 perhaps, the nearest resemblance ; but the Pleura- 

 canthus differed from all the modern sting-rays in 

 having the spine planted at or near the occiput* ^hh^ 

 Portions of petrified bone, of the structure of p . 37 

 shark's spines, graduating into that of the dentine Pleuracanthus 

 and ganoine, of which the marginal denticles are levlsswms - 



i • \i • i + i ■ i , i, ( CoaI ' Dudle ^ 



composed m this and other spines so beset, have 



been obtained from a coal-field in Indiana, U.S. This ich- 



thyodorulite (fig. 38) is remarkable for the large proportional 



* "Annals and Mag. of Nat. Hist." 1857, p. 422. 



