302 Evolution and Distribution of Fishes 



land-bridge of cretaceous-eocene time (Fig. 35, p. 240) to 

 evolve Lepidosiren in South America. 



The Crossopterygii. The groups treated in this and 

 previous chapter, include fishes whose endoskeleton was 

 wholly cartilaginous, or such strengthened by calcareous de- 

 posits. Only the teeth and their accessory parts, or the ex- 

 oskeletal structures, were histologically so elaborated as to 

 constitute true bony material. But in now passing to the 

 Crossopterygii, and later to the Actinopterygii, we ascend 

 to groups that steadily assume increasing endoskeletal com- 

 plexity and bony composition. Correlated therewith is a 

 definite plan of membrane ossification for the head, that is 

 possibly foreshadowed in previous groups; also connected 

 mechanical union and bracing of the anterior paired and of 

 the unpaired fins with the head and the notochordal tube ; in- 

 creased ossification of this tube, and a highly pliable adap- 

 tation of the osseous centres or vertebrae of it, so as to 

 connect and correlate the muscle masses in their motor 

 action; finally, a perfected mode of aquatic respiration that 

 was adapted to proper aeration of the tissues and removal 

 of waste products, from organisms whose motions became 

 steadily more' lithe, and adaptable. 



The above combined characteristics became progres- 

 sively more pronounced from the Mid-devonian on to Juras- 

 sic times, when the Crossopterygii reached or had passed 

 their climax of most perfect evolutionary adaptation; the 

 Polypteridae being now the only surviving remnants. The 

 Actinopterygii however carried forward these details to 

 greatest perfection, and in the process ultimately evolved 

 the highest or teleost division. 



Of the Crossopterygii the most ancient and primitive 

 in their structural details seem to be the Osteolepidae. The 

 family can be traced back to the Lower Old Red Sandstone 

 in such genera as Osteoleph (Fig. 9b, p. 119), Thursiiis 

 and Diplopterus. For in the diphycercal to heterocercal tail, 

 in the rhomboidal and slightly overlapping to cycloid and 

 deeply overlapping scales, in the simple or only slightly 

 infolded wall that covers the pulp cavity of the teeth, as 

 well as other characters, the Osteolepidae and specially 

 Diplopterus seem to be the most primitive. As these 



