“If we include in a definition only those characters which are invariable, the Ganoids may be shortly defined as being those Fish with numerous valves to the stem of the aorta, which is also provided with a muscular coat; with free gills and an operculum, and with abdominal pelvic fins.”
To these distinctive characters, he adds in an appendix to his paper, the presence of the spiral valve, and the absence of a processus falciformis and a choroid gland.
To the distinctive set of characters given by Müller we may probably add the following:—
(1) Oviducts and urinary ducts always unite, and open by a common urinogenital aperture behind the anus.
(2) Skull hyostylic.
(3) Segmentation complete in the types so far investigated, though perhaps Amia may be found to resemble the Teleostei in this particular.
(4) A pronephros of the Teleostean type present in the larva.
(5) Thalamencephalon very large and well developed.
(6) The ventricle in the posterior part of the cerebrum is not divided behind into lateral halves, the roof of the undivided part being extremely thin.
(7) Abdominal pores always present.