4 Memoir Sears Foundation for Marine Research 



Dipnoi and Teleostomii, combined, of Berg Qf)\ the Teleostea of Fowler {2j: 53); 

 and the Class Osteichthyes of Bertin and Arambourg (jj: 1978). 



The Class Osteichthyes as defined above includes all of the living groups of fishes 

 apart from the elasmobranchs (sharks and batoids) and chimaeroids. The cyclostomes 

 are not regarded here as "fishes" in the usual sense of the word. 



Bony Fishes {Osteichthyes') Compared with Cartilaginous Fishes (Chondrichthyes). All 

 living bony fishes differ fundamentally from all living cartilaginous fishes in that the 

 internal skeleton of the former consists of true bone in greater or lesser amount, or of 

 some derivation of true bone, whereas the skeleton of living elasmobranchs and chimae- 

 roids contains no true bone. Bony fishes differ to an even greater extent from the cy- 

 clostomes in that the former have well-developed jaws and (typically) two olfactory 

 organs. The bone of the Osteichthyes represents in part the ossification of the primary 

 skeletal cartilages by the action of bone-forming cells from without (for details, see 

 Goodrich, ^5: 65—67); but in part, especially in the regions of the skull and pectoral 

 arch, it consists of so-called membrane bone, which is not preformed in cartilage. In 

 neither case is it derived from calcifications of the kind that stiffen the cartilaginous 

 skeletons of elasmobranchs and chimaeroids. 



Due to the presence of bone, the skull of all living bony fishes is marked by su- 

 tures,^" which is not so in the cranium or brain case of living cartilaginous fishes. How- 

 ever, this seems an appropriate place to note that in fossil placoderms, which are only 

 remotely related to the bony fishes, the internal skeleton was more or less bony; 

 many of them were more or less completely armored with bony plates, and their skulls 

 showed sutures corresponding to those in Osteichthyes. Also, many of the fossil Ag- 

 natha, of which the cyclostomes of today are commonly regarded as degenerate descen- 

 dants, were variously encased in bony armor, and at least in some cases their skulls 

 were marked with sutures. i' 



Hardly less diagnostic for most of the living bony fishes, including the living 

 coelacanths,^* is the presence (typically) in some part of their fins of segmentally jointed 

 rays'^ (mostly branched) which are either bony or fibrous. These are the soft rays known 

 to every student of fishes. Even among the living Dipnoi (lungfishes), some of the 

 fin rays (hair-fine in this group and of doubtful homology) are segmented, though most 

 of them continue unsegmented throughout life, thus resembling superficially the horny 

 rays (ceratotrichia) of sharks (for details, see Goodrich, 28: 480—482). In the fossil 

 Dipnoi, however, all of the fin rays were segmented. 



No living elasmobranch or chimaeroid has in its fins any structures that correspond 

 structurally to the segmented fin rays of bony fishes. Thus the horny fin rays of sharks 

 are not jointed, and they grow inward from the outer margin of the fin, not outward 



10. The bony neurocranium of the extinct palaeoniscoids, a primitive group of bony fishes, is not marked with sutures 

 (Stensio, 83; Rayner, 65: 287). 



11. For a readable account of these ancient groups, see Romer (73: 25, 38). 



12. For the fin rays of Latimeria, see Millot and Anthony (jj: 2567, 2570, fig. 1840). 



13. The fin rays are not jointed in the lyomerids, the trachypteroids, the stylophoroids, and perhaps in other 

 groups as well. 



