210 Notes on Acanthodian Sharks 
in the Edinburgh Collection), teeth arranged in a graded series with 
stouter ones at the side and with longer and narrower ones in front 
(similar, accordingly, to the conditions in Cladoselache and many mod- 
ern sharks); the spaces between the teeth, however, appear somewhat 
irregular (artifact?). In Fig. 2, in a similar specimen, the heavier 
teeth show lateral cusps, distinctly cladodont in type. On each side of 
the jaw there can be counted about a dozen marginal teeth. There is 
also shown in this specimen (noted for the first time in Acanthodians) 
a second or successional row of teeth, suc; these are distinctly smaller 
than the functional teeth, and cannot be mistaken for the elements of 
the opposite half of the mandible. In Fig. 3 (British Museum, P. 6996) 
is shown a mandible of Ischnacanthus which illustrates strikingly the 
shape and prominence of the lateral teeth. In Fig. 4 (Edinburgh) there 
appear again the prominent lateral teeth, but with indications that their 
bases were somewhat as in other sharks,—not fused (as Smith Woodward 
and others believed) with the ‘‘ membranous calcifications” of both 
jaws. The separateness of the teeth is also indicated in similar specimens 
(I. gracilis, in Edinburgh), shown in Figs. 5-8; in the first of these the 
bases of the separate teeth are particularly well shown, including the 
interlap of the bases, which here occurs very much after the fashion of 
certain modern sharks.’ Interesting, also, is the arrangement of minor 
cusps on either side of the major one, which suggests clearly the teeth 
of Cladodonts. Strikingly Cladodont, moreover, are the various forms 
of teeth occurring in different regions of the mouth. Thus, the form of 
tooth shown in Fig. 9 occurs more nearly symphyseal than the form in 
Fig. 6, and this in turn precedes that of Fig. 7; in Fig. 8 is a detail of 
a favorably preserved series of lateral teeth. In the matter of the suc- 
cessional series of teeth no more definite result could be obtained than 
that shown in Fig. 2; 7. e., indicating a replacement of teeth, and show- 
ing in a more marked way than in Cladodonts or in modern sharks, a 
difference in size between the old and the new teeth.” The growth of the 
new teeth, we therefore infer, was conditioned quite differently than in 
the dental fold of later sharks. A single specimen (Edinburgh), Fig. 10, 
confirms the foregoing account of dental succession; it preserves, it is 
1Cf. especially Figs. 6 and 8. 
2There is accordingly a suggestion that in this order of sharks a dental 
trench and a successional series of teeth had not been perfectly attained; in 
the event of the loss of its large teeth, such an Acanthodian as J. gracilis 
would evidently have to depend upon the small teeth in the second row. That 
these teeth could then increase in size and finally equal the large teeth remains 
an open question; but it is clear that teeth could not be replaced as readily 
as in a modern shark. 
