458 WILLIAM K. GREGORY 



mitted without loss of power to the dermal fins. In general 

 among vertebrates, as specialization for easy swimming progresses, 

 the sources of movement become more deeply seated, and the extent 

 and mobility of the freely flapping membrane increase. This law is 

 illustrated among fishes, marine reptiles, and aquatic mammals. 



Boulenger, followed by Bridge, 1 includes the Rhipidistia and 

 Actinistia under the "suborder" Osteolepida (here taken as an 

 order), with the following definition: 



"The obtusely or acutely lobate pectoral fins articulate with the pectoral 

 girdle by a single basal endoskeletal element. Nostrils on the ventral 

 surface of the snout. Two dorsal fins and an anal fin. Dermal bones of 

 the ethmoid region often fused with one another and with the premaxillae 

 in front and the frontals behind to form a continuous rostral shield. 

 Infra-dentary bones may be present. A series of lateral jugular plates 

 often present in addition to the pair of principal plates." 



In contrast with this the surviving order Cladistia (including 

 only the Polypteridse) is thus defined : 



"Pectoral fins uniserial and abbreviate, with three basal endoskeletal 

 elements. Nostrils on the upper surface of the snout. Entire skeleton 

 well ossified. Notochord replaced by bony amphiccelous vertebral centra. 

 Bones of the ethmoid region not fused to form a rostral shield. Infra- 

 deutary bones absent. Juglar plates reduced to a single pair of large 

 plates." 



The following analysis of the ordinal characters of the Cros- 

 sopterygii was drawn up by Prof. H. F. Osborn and Dr. J. H. 

 McGregor after Cope and Smith Woodward. 



1 Cambridge Nat. Hist. Vol. Fishes, etc., pp. 477, 481. 



