The Antiarcha have also bone-corpuscles in the plates, which are also enameled. The sense organs occupy open grooves, and the dorsal and ventral shields are of many pieces. The head is jointed on the trunk, and jointed to the head are paddle-like appendages covered with bony plates and resembling limbs. There is no evi dence that these erectile plates are real limbs. They seem to be rather jointed appendages of the head-plate, erectile on a hinge like a pec toral spine.
are homologous with the fins and fin-supports of true fishes, or even of sharks. In this group are four well-marked orders, Heterostraci, Anaspida, Aspidocephali and Antiarcha.
The Heterostraci ( irepoc , different; 80TPagov, box) have no bone-corpuscles in the coat of mail. This order includes the Pteraspidw (ex tinct), Thelodontidw (extinct), Drepanaspicke (extinct) and Psammosteids (extinct).
The Anaspida are more fish-like in appear ance, having the armature of the head not plate-like, but formed of tubercles. There are two families, all of recent discovery, Birkenii da (extinct) and (extinct).
The Aspidocephali (also called Osteostraci) have bone-corpuscles in the shields, and the shield of the back is of one piece, without lateral line-channels or sense-organs. The order includes four families, Afekaspide (extinct), There is but one family, Asterolefrida (ex tinct). Pterichthyodes milleri, named by Agas six for Hugh Miller, from the Lower Devonian, is the best-known species.
Class group of ex tinct mailed fishes is known as Arthrodira (dpepov, joint; dew*, neck). In this group jaws are developed, but of peculiar character. the mandibles being regarded as mere der mal elements, not forming part of the skele ton. The head in all the species is covered with a great bony helmet. Behind this on the nape is another large shield, and between the two is typically a hinge-joint, which has been compared to the hinge of a spring-beetle (dater). Some of these plates are traversed by sensory grooves. Nothing whatever is known of the internal structure, and as the skeleton is soft, the back bone notochordal, there is no trace of shoulder girdle, nor any certain evidence of limbs. although peculiar structures have been inter preted as such. The presence of a peculiar type of jaws separates the group from the mailed ostracophores, from which the arthrodires differ also widely in the character of the armature.
Dr. Woodward and several other recent writ ers have regarded the arthrodires as armored, widely modified offshoots of the primitive Dip neusta. But the evidence does not seem to
justify the union of the arthrodires with the latter group, and it would seem as reasonable to regard them as derived directly from the sharks or the ostracophores. The arthrodiran fishes occur in abundance from the Silurian times to the Mesozoic. In the Devonian their gigantic size and thick armor gave them the leading position among the hosts of the sea, ranging in size that of the perch to that of the bask ing-sha rk.)) The class, called by Dr. Dean Arthrognathi, is divided by him into two Arthro dira, with a hinge at the neck, and Anarthrodira, without hinge. In the first of typical sub-class are two orders, Temnothoracici, with the single family Chelonichthyide (extinct), Anthrotho raci, with the families Coccosteidce, Dinichthyide (extinct), Titanichthyida• (extinct), My lostomida, and Selenosteide (extinct). To Chelonichthyidce belongs the noted species Homosteus or Pterichthyodes milleri, celebrated by Hugh Miller under the name of °the Astero lepis of in his (Footprints of the Creator.
The arthrodires without joint at the neck constitute the order Stegothatami, with the fam ilies of Macropetalichthyide (extinct), and Asterosteicke (extinct).
The best known of the many genera of arthrodires is Coccosteus, found in the Scottish Devonian.
Class Teleostomi.— We may unite the re maining groups of fishes under a single class for which the name Teleostomi ( Taros, true; erhaa, mouth), proposed by Bonaparte in 1838, may be retained. The fishes of this class are characterized by the presence of a suspensorium to the mandible, by the existence of membrane bones (opercles, suborbital, etc.) on the head, by a single gill-opening, leading to gill-arches bearing bilamellate gills, and by the absence of claspers on the ventral fins. The skeleton is more or less ossified in all the Teleostomi. More important as a primary character distinguishing these fishes from the sharks is the presence typically and primitively of the air-bladder. This arises at first as a diverticulum from the ventral side of the (esophagus, and develops as a lung, hut in later forms it becomes degraded to a swim-bladder, springing from the dorsal side of the alimentary canal, and in very many forms it is altogether lost with age. The group comprises the vast majority of recent fishes, as well as a large percentage of those known only as fossils. In these, the condition of the lung can be only guessed.