588 Arthrodires 



shield deeply concave on lateral margins, no orbital rim here ap- 

 parent. Nuchal border deeply indented. (Centrals separate from 

 marginals.) Cranio-dorsal hinges large in size. Dorsal armoring 

 reduced antero-posteriorly, giving an almost zone-like appear- 

 ance. Dorso-median crescent-shaped, with feeble keel and 

 knob." Selenosteus glaber is described by Dean from the Cleve- 

 land shales. 



Relations of Arthrodires. To complete our account of the 

 Arthrodira we may here summarize Dr. Dean's reasons for separat- 

 ing its members from true fishes on the one hand and from the 

 Ostracophores on the other. 



" FIRST. The Arthrodira cannot be strictly included among 

 the Pisces. According to the definition of the latter class its 

 members are Craniotes possessing the following characters: 

 a, dermal defenses which in their simplest terms can be re- 

 duced to the shagreen denticles of the Elasmobranch ; b, a 

 series of definite gill-arches whose foremost elements are meta- 

 morphosed into hyoid and mandibular apparatus; c, paired 

 fins, or their equivalents. In the first of these regards I think 

 it can be shown that the remarkable character of the dermal 

 plates in the Arthrognaths approaches rather that of the Ostra- 

 cophores than that of the Pisces. In certain of these forms, 

 Trachosteus, for example, the tuberculated plates are made up 

 of inner and outer elements, each with tubercles, which denote 

 a distinctly different mode of origin from that of any known 

 type of fish. The absence of remains of gill-arches in the Ar- 

 thrognaths would be not a series objection to including these 

 forms among Pisces, especially in view of the fact that carti- 

 laginous gill-arches are rarely preserved even in favorable 

 fossils. But that their presence is more than doubtful is in- 

 dicated by the peculiar character of the 'jaws' in these forms. 

 For the character of these structures is such as to suggest that 

 they are not homologous with the branchial-arch jaws of the 

 true fishes, but are rather parallel structures which owe their 

 origin to distinctly exoskeletal elements, i.e., that they were 

 derived from dermal plates surrounding the mouth, which be- 

 came mobile, and whose edges became apposed as sectorial 

 structures. I would in this connection call attention to the 

 fact that the 'mandibles,' ' premaxillary, ' and 'maxillary' 



