Fig. 363.—Pterichthyodes testudinarius Agassiz, side view. (After Zittel, etc.)
Fig. 364.—Birkenia elegans Traquair. Upper Silurian. (After Traquair.)
Order Anaspida.—Recently a fourth order, Anaspida (ἄ, without; ἀσπίς, shield), has been added to the Ostracophori through the researches of Dr. Traquair. This group occurs in the Upper Silurian in the south of Scotland. It includes the single family Birkeniidæ, characterized by the fusiform body, bluntly rounded head, bilobate, heterocercal tail, and a median row of hooked spinous scales along the ventral margin. No trace of jaws, teeth, limbs, or internal skeleton has been found. Unlike other Ostracophores, Birkenia has no cranial buckler with orbits on the top, nor have the scales and tubercles the microscopic structure found in other Ostracophores. In the genus Birkenia the head and body are completely covered by tubercular scutes. The gill-openings seem to be represented by a series of small perforations on the sides. A dorsal fin is present. Birkenia elegans is from the Ludlow and Downstonian rocks of southern Scotland. Lasianius problematicus from the same rocks is very similar, but is scaleless. It has a row of ventral plates like those of Birkenia, the only other hard parts it possesses being a number of parallel rods behind the head, homologous with the lateral series of Birkenia. Lasianius is therefore a specialized and degenerate representation of Birkenia, differing somewhat as "the nearly naked Phanerosteon differs from other Palæoniscidæ whose bodies are covered with osseous scales."
Fig. 365.—Lasianius problematicus Traquair. Upper Silurian. (After Traquair.)
FOOTNOTES:
[152] This group was first called by Cope Ostracodermi—a name preoccupied for the group of bony trunkfishes, Ostracidæ. The still earlier name of Placodermi, chosen by McCoy (1848), was intended to include Arthrodires as well as Ostracophores. Rohon (1892) calls the group Protocephali, and to the two orders he assigns the names Aspidorhini and Aspidocephali. These groups correspond to Heterostraci and Osteostraci of Woodward. Another name of early date is that of Aspidoganoidei, given by Professor Gill in 1876, but not defined until 1896. These fishes are, however, not "Ganoids" and the name Ostracophori seems to receive general preference. The group Peltacephalata of Patten corresponds essentially to Ostracophori, as does also the order Hypostomata of Gadow.