THE GANOIDEI. 129 



laspidm would approach the Marsipobranchii more nearly than 

 any of the other amphirhine fishes do. 



6. The JPlacodermi, comprising the genera Coecosteus, 

 Pterichthys, Asterolepis, and some others, are known to occur 

 only in the Devonian and Carboniferous formations. In these 

 fishes the pectoral region of the body is encased in great bony 

 plates, which, like those of the skull, are ornamented with 

 dots of enamel. The caudal region was covered with small 

 scales in Pterichthys, while in Coccostevs it appears to have 

 been naked. The pectoral member of Pterichthys is exceed- 

 ingly long, covered with suturally-united bony plates, and 

 united with the thoracic plates by a regular joint. In Coccos- 

 teits the pectoral member seems to have had the ordinary con- 

 struction. The bones of the head and thorax of Goccosteus 

 nearly resemble those of certain Siluroid fishes (e. g., Clarias) 

 in their form and arrangement, and it seems probable that the 

 Placodermi were annectent forms between the physostome 

 Teleostei and the Ganoidei. 



t. The A-canthodidcB, on the other hand, seem to have con- 

 nected the Ganoidei with the Elasmohranchii. The scales 

 of these fishes of the Devonian and Carboniferous formations 

 are very small, and similar to shagreen ; spines, resembling the 

 dermal defences of the Elasmohranchii^ are placed in front 

 of more, or fewer, of the median and of the paired fins. The 

 skull appears to have been unossified, and the pectoral arch 

 seems to have consisted of a single bony hoop. 



The Pycnodontidce, which are commonly grouped among 

 the Ganoids, are fishes with much-compressed bodies, like the 

 John Dory or the Filefishes, covered with large rhomboidal en- 

 amelled scales, from which bony ridges projected internally, 

 and were imbedded in the integument. The notochord is per- 

 sistent, but the neural arches and the ribs are ossified. The 

 proximal ends of the ribs, imbedded in the sheath of the noto- 

 chord, are but little expanded in the more ancient members 

 of the group, while, in the more modern species, they enlarge, 

 and at length unite by serrated sutures, giving rise to spurious 

 vertebrse. The skull is high and narrow, as in JBalistea ; the 

 preraaxillas are small, and there are no teeth in the maxillae, 

 but several longitudinal series of crushing teeth (the vomer 

 and parasphenoid ?) are attached to the base of the skull. 

 These bite between the rami of the mandible, which are also 

 armed with several rows of similar teeth. The teeth of the 

 Pycnodonts have no vertical successors. The pectoral fins are 

 small, the ventral, obsolete. The Pycnodonts are all extinct, 



