The family of Dinichthyidæ consists of "Arthrothoraci with stout trenchant jaws, whose cutting surfaces have worn away marginal teeth. Plates heavy. Head-shield with conspicuous lateral indentation to form dorsal border of orbit. Preorbitals separated by rostral and pineal elements, the latter passing backward between the anterior ends of the centrals. Cranio-dorsal joint conspicuous. Median dorsal shovel-shaped, nearing a stout keel with a large neck and with heavy gouge-shaped terminal. Postero-dorso-lateral relatively small in size." Dinichthys hertzeri and numerous other species are described from the Devonian and Carboniferous rocks of Ohio.
The Titanichthyidæ are "Arthrothoraci with slender edentulous jaws bearing a longitudinal sulcus. Plates squamous. Head-shield wide, with indentations to form dorsal border of orbit. Cranio-dorsal joint complete, but of relatively small size. Median dorsal with lateral border indented with rudimentary keel and with flat and rounded terminal. Antero-dorso-lateral with an area of overlap on median border." Titanichthys agassizi is a gigantic mailed fish from the Lower Carboniferous of Cleveland, Ohio.
The Mylostomidæ are "Arthrothoraci with dental elements in the character of crushing plates. Cranial shield wide, rounded anteriorly, deeply indented in nuchal margin; orbital rim not apparent in dorsal aspect. Central separated from marginal." Mylostoma terrelli is based on jaws from Cleveland, Ohio.
The Selenosteidæ are "Arthrothoraci with jaws studded with cuspidate teeth; the mandibular rami rounding out anteriorly or presenting diverging tips, bearing teeth in the symphysis. Cranial shield deeply concave on lateral margins, no orbital rim here apparent. Nuchal border deeply indented. (Centrals separate from marginals.) Cranio-dorsal hinges large in size. Dorsal armoring reduced antero-posteriorly, giving an almost zone-like appearance. Dorso-median crescent-shaped, with feeble keel and knob." Selenosteus glaber is described by Dean from the Cleveland shales.
Relations of Arthrodires.—To complete our account of the Arthrodira we may here summarize Dr. Dean's reasons for separating 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 reduced to the shagreen denticles of the Elasmobranch; b, a series of definite gill-arches whose foremost elements are metamorphosed 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 Ostracophores 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 Arthrognaths would be not a serious objection to including these forms among Pisces, especially in view of the fact that cartilaginous gill-arches are rarely preserved even in favorable fossils. But that their presence is more than doubtful is indicated 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 became 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' dental plates[159] were not fixed in the sense in which these elements are in the true Pisces. On the evidence of several types, Dinichthys, Titanichthys, Mylostoma, Trachosteus, Diplognathus, and other of the American forms, Macropetalichthys[160] excepted, there is the clearest proof that each element of the jaws had a considerable amount of independent movement. On account of the mobility of these elements the name Arthrognathi is suggested. Thus the mandibular rami could change the angle of inclination towards each other, as well as their plane with reference to the vertical axis. So, too, could the 'premaxillæ' be protracted like a pair of bent fingers, and it is more than probable that the 'maxillæ' had a considerable amount of independent movement. In connection with these characters it is also important to note that the blades of the 'mandible' show nowhere the faintest trace of an articular facet for attachment to the cranium. In short, the entire plan of the mandibular apparatus in these forms is strikingly unfish-like, although one will frankly confess that it is remarkable that these forms should have paralleled so strikingly the piscine conditions, to the extent of producing mandibular rami margined with teeth, and an arrangement of toothed elements on the 'upper jaw' which resembles superficially the premaxillary and maxillary structures of teleostomes, or the vomero-palatine structures of lung-fishes and chimæras.
"In the matter of paired fins there seems little evidence to conclude that either pectoral or pelvic fins were present. In spite of the researches upon these forms during the past half-century, no definite remains of pectoral fins have been described. The so-called pectoral spines described for Dinichthys by Newberry, whatever they may be, certainly are not, as far as the present evidence goes, pterygial, nor are the similar structures in Brachydirus.[161] The sigmoid element, described as a 'pelvic girdle' by Smith Woodward, in Coccosteus, a structure which appears to occur in a small species of Dinichthys(?), may as reasonably be interpreted as a displaced element of the armor-plates of the trunk. In Coccosteus, as far as I am aware, it occurs in well-preserved condition in but a single specimen.
"In referring to the singular joint between the shoulder-plates and the hinder margin of the cranium Smith Woodward has called attention to one of the striking features of the group. It is one, however, which, as a functional structure, i.e., a joint, characterizes only a portion of its members; and in these the region in which vestiges of the joint are sought is overlaid and concealed by dermal plates. Such are the conditions in Macropetalichthys (with transitional characters in Trachosteus and in Mylostoma). For this form a special subclass (or order) may be created which we may term Anarthrodira.
"Second. The Arthrognathi cannot well be included in any other class. It would certainly be more convenient to retain the Arthrognaths among the Ostracophores, regarding them as a fourth subclass, were it not that they differ from them in so marked a way in the presence of well-marked vertebral arches, of supports for the unpaired fin, and in the possession of 'jaws.' In these regards—add to them the (probable if not certain) absence of the paired paddle-like 'spines'—they stand certainly further from the Antiarcha than these from the Osteostraci, or than the latter from the Heterostraci. It appears to me desirable, therefore, that the Arthrodira and the Anarthrodira be brought together as a separate class. Should subsequent researches demonstrate a closer affinity with the Ostracophores, the Arthrognathi can be regarded as of rank as a subclass, with the orders Anarthrodira and Arthrodira."[162]