602 The Crossopterygii 



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 



FIG. 372. Polyplerus 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 Polypterida, 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 

 Tarrassiida 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 Rhipidistia 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, 

 Holoptychiidcz, Megalichthyida, Osteolepidce, and Onychodontida, 

 the first of these being considered as the nearest approach of 

 the Crossopterygians to the Dipnoans. 



