Bashford Dean Palla 
true, only a fragment of the jaw, showing in visceral aspect the region 
of its articular margin. Here it presents a series of enlarged denticles 
which pass from the condition of shagreen tubercles into the marginal 
teeth ; but it will be seen that as the submarginal denticles pass forward 
and alee toward the rim of the jaw, they become reduced to a single 
successional row, as already noted in Fig. 2. 
In the genus Acanthodopsis a remarkably perfect dentition is shown 
in a specimen in the British Museum (Fig. 11). In each ramus there 
was present a row of a dozen (possibly there were several more) teeth 
which increase in size as they pass toward the region of the symphysis. 
They are certainly stouter in this form than in Ischnacanthus, and the 
symphyseal teeth were as conspicuous, or even more conspicuous, than 
the similarly situated teeth in a modern Odontaspid shark. Moreover, 
that they were here functionally important seems evident from the thick- 
ness of the anterior reach of the meckelian cartilage. 
From the evidence of dentition, accordingly, Acanthodians, in certain 
of the genera at least, resemble Devonian Cladodonts, and from this 
structural standpoint, there is suggested a closer kinship between the 
groups.” 
’ That the Acanthodians were in a general sense planktonophagous, as 
Dollo has recently suggested, ’96, on the ground that in several genera teeth 
have not been described, is by no means evident. It is certainly possible, in 
view of the evidence that a widely diversified evolution took place within this 
group, that some of its members were specialized to secure plankton, just as 
were members of other optimum groups, euselachians, ganoids, teleosts. It 
is, however, clearly safer to conclude, as has hitherto been done, that the 
Acanthodians in which the dentition was reduced (or rudimentary) were 
forms whose diet was restricted to small or minute organisms. To specify 
that this food was plankton, and that the structures of the fish were typically 
planktonophilous, carries one beyond the limits of evidence. Nor does it 
seem safe to assume, as Dollo has done, that those Acanthodians which were 
not of this form were developed with an abyssal type of dentition, after the 
fashion of Stomias, Aulops, ete., seems equally wide of the mark. The 
present notes indicate a rather normal type of selachian dentition, primitive, 
we infer, in as much as the mouth-invading shagreen seems to have gone no 
farther than the mouth rim,—there being no evidence of a wide successional 
series of teeth as in euselachii. And we can assume, accordingly, that in the 
Acanthodian, as in other fishes, Characinids for example, large teeth predicate 
large prey and rapacious habits. But not necessarily bathybial. There is 
evidence on the other hand that Acanthodians could not have inhabited deep 
water, for the rule is fixed that bathybial forms, on account of the mechani- 
cal conditions of their habitat, are extremely defective in hard structures, 
scales, bones, and spines. The stout-walled spines and dense shagreen of 
Acanthodians should alone have precluded the assumption that uuey were 
deep-water forms. 
