The Asterosteidæ perhaps constitute a second family in this order. The single species Asterosteus stenocephalus is from the Devonian of Ohio.
Arthrodira.—"Arthrognaths in which the dorsal armoring is separated into dorsal and cranial elements, the latter attached to the former movably by means of a pair of peg-and-socket joints. The interval lying between cranial and dorsal armoring does not appear to have been protected by plates, and in the median line, instead of the cranio-central of the Anarthrodires, there are separate elements, median occipital, median dorsal, and perhaps others. Suborbital plates present. Jaws of three pairs of elements. Ventral armoring of two pairs of lateral and two median elements."
Temnothoraci (τέμνω, to cut; θώραξ, thorax).—"Arthrodires whose cranial and dorsal shields are closely apposed, separated only by a transverse fissure-like interval (whence the ordinal name); interarticulation of cranial and dorsal shields little developed. Head-shield elliptical in outline as far as the line of the transverse division. The anterior rim of the shoulder-shield flattened at its sides, suggesting a rudiment of the vertical partition of the Anarthrodira. Suborbital plate is present, but takes no part, apparently, in the ventral boundary of the orbit, this being formed, as in the Anarthrodira, by the pre- and postorbital elements. Jaws, ventral armoring, and endoskeleton not definitely known."
One family, Chelonichthyidæ, thus defined:
"Temnothoraci with orbits relatively small in size and situated well forward in the head-shield. Occipital elements produced antero-posteriorly, the external occipital forming the posterior lateral angle of the head, no projection of the head occurring in the region of the marginal plate. Median occipital trapezoidal. Centrals take part in the median boundary of the orbits, and embrace the pineal plate. Median dorsal with poorly developed keel and terminal process."
Heterosteus asmussi (perhaps to be called Ichthyosauroides spinosus) is a gigantic species from the Lower Devonian of Livonia.
Allied to this species is Homostius milleri from Scotland, celebrated as the "Asterolepis of Stromness" in Hugh Miller's "Footsteps of the Creator." Another notable species is Homostius formosissimus from the Lower Devonian of Russia.
Arthrothoraci.—"Arthrodires whose dorsal shield articulates with the head-roof by a conspicuous and movable peg-and-socket joint, and leaves a definite interval (unprotected?) between the two armorings. Orbits marginal, bounded inferiorly not by the suborbital element. In the head-shield the postero-lateral angles formed by the marginal plate (Phlyctænaspis?), the occipital border concave. A dorsal fin is present, supported by endoskeletal elements." Five families, the most important being the Coccosteidæ, thus defined:
"Arthrothoraci with head-shield hexagonal in outline. Median occipital trapezoidal, margins underlapped conspicuously by the external occipitals. Prefrontals meet below pineal plates, thus occluding this element from contact with centrals. The median dorsal plate elongated, terminating in an acute heavy point; no definite ventral keel; its anterior border approaches the head-shield more closely than in related families. Cranio-dorsal joint relatively small. Postero-dorso-lateral large." (?A pair of spines occurs in the pectoral region.) The best-known species is Coccosteus cuspidatus (decipiens) of the Lower Red Sandstone or Devonian of Scotland.