Order Antiarcha.—The Antiarcha (ἀντί, opposite; ἀρχός, anus) 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 evidence that these erectile plates are real limbs. They seem to be rather jointed appendages of the head-plate, erectile on a hinge like a pectoral spine. There are traces of ear-cavities, gill-arches, and other fish-like structures, but nothing suggestive of mouth or limbs.
This group contains one family, the Asterolepidæ, with numerous species, mostly from Devonian rocks. The best known genus is Pterichthyodes,[156] in which the anterior median plate of the back is overlapped by the posterior dorso-lateral. Pterichthyodes milleri from the Lower Devonian, named by Agassiz for Hugh Miller, is the best known species, although numerous others, mostly from Scottish quarries, are in the British Museum. Asterolepis maximus is a very large species from the same region, known from a single plate. Bothriolepis canadensis is from the Upper Devonian of Scaumenac Bay near Quebec, numerous specimens and fragments finely preserved having been found.
Fig. 361.—Cephalaspis dawsoni Lankester. Lower Devonian of Canada. Family Cephalaspidæ. (After Woodward.) In the square a portion of the tubercular surface is shown.
Microbrachium dicki with the pectoral appendages small occurs in the Devonian of Scotland.
The earliest remains of Ostracophori are found in Ordovician or Lower Silurian rocks of the Trenton horizon at Cañon City, Colorado. These consist of enormous numbers of small fragments of bones mixed with sand. With these is a portion of the head carapace of a small Ostracophore which has been named by Dr. Walcott Asteraspis desiderata and referred provisionally to the family of Asterolepidæ, which belongs otherwise to the Lower Devonian.
Fig. 362.—Pterichthyodes testudinarius (Agassiz), restored. Lower Devonian Family Asterolepidæ. (After Traquair and others.)
With these remains are found also scales possibly belonging to a Crossopterygian fish (Eriptychius). These remains make it evident that the beginning of the fish series lies far earlier than the rocks called Silurian, although fishes in numbers are not elsewhere known from rocks earlier than the Ludlow shales of the Upper Silurian, corresponding nearly to the Niagara period in America.
In the Ludlow shales we find the next appearance of the Ostracophores, two families, Thelodontidæ and Birkeniidæ, being there represented.