6 Memoir Sears Foundation for Marine Research 



in elasmobranchs and chimaeroids the so-called placoid scales are tooth-like in structure, 

 have an extensive pulp cavity, and are tipped with an enamel-like substance of ectodermic 

 origin. Furthermore, the scales of a shark are not only short-lived but increase in number 

 as it grows. Although the presence of bony-fish scales is positively diagnostic for those 

 fishes that have them, their absence is not negatively diagnostic, for it is common 

 knowledge that scales are wholly lacking in some bony fishes. Furthermore, certain 

 groups of the bony fishes have placoid denticles of the elasmobranch type on their 

 bony scales, e. g. the polypteroids, the lepisosteids (gars), and some ostariophysids (cat- 

 fishes); and some of this last group have placoid denticles on their fin spines as well.^* 

 Denticles, believed to be placoid in nature (an interpretation calling for verification), 

 have also been described for the snouts of young Xiphias (swordfish) and young Istio- 

 phorus (sailfish) (jj: 321, pis. 1,2); and Dr. Ethelwynn Trewavas informs us that they 

 are present on the sucking disc of remoras as well (personal communication). 



It has been stated repeatedly that, whereas the teeth of elasmobranchs are simply 

 implanted in the gums, those of most bony fishes are set in the bone. The polyodontids 

 (paddlefishes), however, and certain Salariinae (blennies) parallel the elasmobranchs in 

 this respect, their teeth having no special connection with the jaw bones; and the teeth 

 in Plecoglossus (related to the salmonoids) are attached to the jaws by connective tissue 

 only {14: 437). 



The nasal openings are single on either side among the elasmobranchs in general 

 and among the chimaeroids, but among bony fishes they are double (typically) on each 

 side. However, some bony fishes, e. g. pomocentrids and cichlids, have only one nasal 

 opening on each side. On the other hand, it has been discovered recently that each 

 nasal opening in one genus of torpedo rays {Diplohatis) is divided into two {^8'. 482, 

 fig. 2; 7: 562, 563, fig. 7). 



Various texts have emphasized the mode of suspension of the pectoral girdle as a 

 diagnostic feature: it is attached to the skull In typical bony fishes; it is not connected 

 at all to the axial skeleton in living sharks and chimaeroids; and it is attached to the 

 anterior part of the vertebral column in batoids. However, among bony fishes, the 

 pectoral girdle has lost its attachment to the skull in the deep-sea giganturoids and sac- 

 copharyngoids, in the amphipnoids (a group that is eel-like in appearance but differs 

 widely from the true eels in anatomy), and in the Apodes (eels) as a whole; and in some 

 Apodes (Muraenidae) the pectoral girdle is lacking altogether. On the other hand, the 

 girdle is attached by a ligament to the third or fourth vertebra in the Opisthomi.^" 



The presence of either a swim bladder or a functional lung is diagnostic for such 

 of the bony fishes as have either, unless a small diverticulum from the dorsal side of 

 the oesophagus, observed in embryos of certain sharks by Miclucho-Maclay {^g: 448, 

 pi. 5), represents the rudiment of a swim bladder. But the absence or presence of a 



19. For illustrations conveniently available of placoid denticles among bony fishes, see especially Goodrich, in Lan- 

 kester (^5: 231, fig. 201; 289, fig. 262; 291-292, figs. 264, 265). For a recent account, with references and excellent 

 illustrations of these denticles among living catfishes, see 0rberg {$6: 487, figs. 4 C, 40). 



20. An Order of eel-shaped freshwater fishes of doubtful affinity, known from tropical Africa, the Euphrates, and 

 southern Asia north to Peking (65: 219). 



