28 SIRENOID AND CROSSOPTERYGIAN GANOIDS. 



random one out of this long list, and extend its signification so as to include under it all 

 the rest. I might, however, have adopted Agassiz's little-descriptive name deedaleus, 

 had the type been discoverable. The figure 1 leaves room for a doubt respecting the 

 specific identity of this form with the rest, though the probabilities are much against its 

 distinctness. I have at length decided to give to the new species, under which most of 

 those named by Agassiz are now placed, the name polymorphus, which Sir Philip Egerton 

 suggests as appropriate to so variable a series. For the second species Agassiz's name 

 parvus may be retained, though the figure 2 is not very distinctive. 3 



While advocating the use of these two specific names, I think it must be admitted 

 that no species of fishes can be regarded as well established, so long as they rest upon 

 fossil teeth alone, and this distrust is naturally increased when the fossil teeth are, as in 

 the present case, remarkably variable. Indications are not entirely wanting of a bridge 

 between parvus and polymorphus (PI. V, figs. 5, 9 ; PI. IV, fig. 10), and it will be wise to 

 accept such determinations as liable to be upset altogether by future additions to our 

 collections. 



C. polymorphus, n. sp. PI. II, figs. 1 — 13; PI. Ill, figs. 1 «, b, c, 2, 5 a, b, c ; 



PI. IV, figs. 1—11 ; PL V, figs. 1 a, b, 5. 



Large ; variable in form, ranging from triangular to oval ; opposed surfaces generally 

 sinuous, sometimes flat, the mandibular tooth more often concave, if each is simply 

 curved; usually strongly pitted. Denticles generally 5 — 4, the first often prominent, 

 especially in the palatal tooth ; the ridges from the denticles low and ill-defined, not 

 reaching the angle. External margin sinuous, not angulated, usually defined. Thick- 

 ness variable, probably according to age; in presumed young specimens the denticles less 

 prominent. 



PI. II, figs. 5, 10, 11, 13 (mandibular), and PL IV, figs. 1, 9, PL V,fig. 1 (palatal), 

 are extreme forms, but connected with such more ordinary teeth as PL II, figs. 2, 3, 4, 

 6 (mandibular), PL III, fig. 5, PL IV, fig. 3 (palatal), by many gradations. PL III, 

 fig. 1, connects this species with C. runcinatus, Plieninger. 



It must not be forgotten that the teeth furnish the only specific characters of the 

 fossil Ceratodontes, and that these are highly special as to number, size, and form. It 

 may well be that they varied more conspicuously than other parts of the body, like the 



1 ' Poissons Fossiles,' t. iii, pi. xx, fig. 1G. 



2 Ibid., fig. 1. 



3 It is satisfactory to know that the great reduction of species here proposed has the general 

 approval of Mr. E. B. Tawney, the custodian of the Bristol fossils, who has long held that the Protean 

 Ceratodonts of Aust Cliff were reducible to a very few variable species. I have to thank Mr. Tawney for 

 valuable suggestions, as well as for much friendly help in studying, describing, and figuring the collection 

 under his care. 



