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184 The True Sharks 
Order Notidaai—We may recognize as a distinct order a 
primitive group of recent sharks, a group of forms finding its 
natural place somewhere between the Cladoselachide and Hetero- 
dontide, both of which groups long preceded it in geological 
time. 
It has been lately announced that a ruaimentary sixth gill- 
arch exists in Heterodontus. This would show the close affinity 
of these two primitive groups, Notidani and Cestraciontes, and 
the latter should be removed from the Asterospondyli, The 
presence of five species in the Squalide perhaps indicates affinity 
with Heterodontus. The fact that Cestractontes were the only 
sharks living in the Triassic, soon followed by Notidani and later 
by squaloid and galeoid sharks, seems to be significant. 
The name Notidani (Notidanus, vwridavos, dry back, an old 
name of one of the genera) may be retained for this group, 
which corresponds to the Diplospondyli of Hasse, the Opis- 
tharthri of Gill, and the Protoselachtt of Parker and Haswell. 
The Notidani are characterized by the primitive structure of 
the spinal column, which is without calcareous matter, the centra 
being imperfectly developed. There are six or seven branchial 
arches, and in the typical forms not in Chlamydoselachus) the 
palato-quadrate or upper jaw art-culates with the postorbital 
Fic. 126. —Griset or Cow-shark, Hexanchus griseus (Gmelin). Currituck Inlet, N.C. 
region of the skull. The teeth are of primitive character, of 
different forms in the same jaw, each with many cusps. The 
fins are without spines, the pectoral fin having the three basal 
cartilages (mesopterygium with propterygium and metapte- 
rygium) as usual among sharks. 
The few living forms are of high interest. The extinct species 
are numerous, but not very different from the living species. 
