Mr. Regan especially calls attention to the very close similarity in structure of pectoral and ventral fins in the Chondrostean Ganoids, Psephurus and Polyodon, with that of the anal fin in the same fishes. From this he derives additional evidence in favor of the origin of paired fins from a lateral fold. In his view, the Chondrostei have sprung directly, through ancestors of the Lysopteri and Selachostomi, from pleuropterygian sharks (Cladoselache) of the Lower Silurian, and the true fishes on the one hand and the Crossopterygian-Dipneustan-Placoderm series on the other are descended from these. The absence of the lower jaw in fossil remains of Ostracophores may be due to its cartilaginous structure. "There is no justification for regarding the Crossopterygii as less specialized than the Chondrostei because they were the earlier dominant group."
These views are very suggestive and contain at least some elements of taxonomic advance, although few naturalists of to-day will regard the Chondrostean Ganoids as more primitive than the fishes called Crossopterygii and Placoderms.
These conclusions are summarized by Mr. Regan as follows:
(1) The Chondrostei are the most generalized Teleostomi.
(2) The Crossopterygii differ from them
(a) in the lobate pectoral fin;
(b) in the larger paired gular plates.
(3) The Placodermi (Coccosteidæ, Asterolepidæ, Cephalaspidæ) are a natural group, not related to the Heterostraci, which are Chondropterygii. They may probably be regarded as armored primitive Crossopterygii, this view being most in accordance with
(a) the arrangement of the cranial roof-bones in Coccosteus;
(b) the structure of the ventral fin in Coccosteus;