132 
SELACHII. 
The following extinct genera, being known only by the dentition 
and vcrtebraj, are provisionaUy associated with the family of Mylio- 
batidae, Apoeopodon has already been placed here by Cope, and 
the present writer^ has suggested that this genus is not improbably 
a link between the more typical members of the family and the 
Cretaceous Ptychodus. 
Genus APOCOPODON, Cope. 
[Proc. Amer. Phil. Soc. vol. xxiii. 1886, p. 2.] 
Median teeth quadrate in form ; side teeth with irregularly an- 
gnlated lateral extremities. Crown elevated, separated from the 
root by a constriction, the grinding surface covered by a dense layer, 
with fine antero-posterior wrinkles ; attached surface of root with 
few, verjf shallow grooves. 
Apocopodon sericem. Cope, loc. cit., is the only known species, from 
the uppermost Cretaceous of Maria Farinha, Province of Pernam- 
buco, Brazil. 
Genus PTYCHODUS, Agassiz. 
[Poiss. Foss. vol. iii. 183t), p. 150.] 
Syn. Aulodua, F. Dixon, Foss. Suss. I860, p. .360. 
Teeth quadrate in form, with elevated crown, somewhat over- 
hanging, and sharply separated from the root by a constriction. 
The crown is enamelled and ornamented with large transverse or 
radiating ridges, surrounded by a more finely-marked marginal area, 
of greater or less extent. The surface of attachment of the root is 
smooth. 
Fig. 6. 
Diagram of arrangement of teeth of Ptychodus decurreus 
A. Dpperjaw. B. Lower jaw. 
Proc. Qeol. Assoc, rol. x. (1888), p. 207. 
Eedueed from Proc. Geol. Assoc, vol. x. (1888), p. 296. 
