228 The Crossopterygil 
Ganoids may be derived, this giving way in the process of de- 
velopment to the imperfectly homocercal tail of the salmon, 
the homocercal tail of the perch, and the isocercal tail of the 
codfish and its allies, the gephyrocercal and the leptocercal tail, 
tapering or whip-like, representing various stages of degenera- 
tion. Boulenger draws a distinction between the protocercal 
Fic. 161.—Polypterus congicus, a Crossopterygian fish from the Congo River. Young, 
with external gills. (After Boulenger.) 
tail, the one primitively straight, and the diphycercal tail 
modified, like the homocercal tail, from an heterocercal ancestry. 
Orders of Crossopterygians.—Cope and Woodward divide the 
Crossopterygia into four orders or suborders, Haplistia, Rhipi- 
distia, Actinistia, and Cladistia. To the latter belong the exist- 
ing species, or the family of Polypteride, alone. Boulenger unites 
the three extinct orders into one, which he calls Osteolepida. 
In all three of these the pectorals are narrow with a single basal 
bone, and the nostrils, as in the Dipneustans, are below the 
snout. The differences are apparently such as to justify Cope’s 
division into three orders. 
Haplistia—In the Haplistia the notochord is persistent, and 
the basal bones of dorsal and anal fins are in regular series, 
much fewer in number than the fin-rays. The single family 
Tarrassiid@ is represented by Tarrasius problematicus, found 
by Traquair in Scotland. This is regarded as the lowest of the 
Crossopterygians, a small fish of the Lower Carboniferous, the 
head mailed, the body with small bony scales. 
Rhipidistia——In the Rhtpidistia the basal bones of the median 
fins (“‘axonosts and baseosts’’) are found in a single piece, not 
separate as in the Haplistia. Four families are recognized, 
Holoptychiide, Megalichthyide, Osteolepide, and Onychodontide, 
the first of these being considered as the nearest approach of 
the Crossopterygians to the Dipnoans. 
