Editorial Comment. 247 
teeth as in all the other known ostracoderms. But as Prof. 
Traquair points out, if the so-called "fin-fiaps" of Thelodus 
and Lanarkia really acted as fins, they have lost their function 
in Drepanaspis, where they are replaced by unyielding struc- 
tures of bone. In that case another organ must be looked 
for to perform the duty of fins, as it is scarcely likely that 
these creatures could direct their movement without some 
other engine than the propellmg tail. The same argument 
will apply to the rest of the ostracoderms. 
Continuing his remarks along the same line, Prof. Tra- 
quair claims the curious fossils, known as Psammosteidae for 
the ostracoderms. These have been referred to the coela- 
canths, to the coccosteids and to the supposed "armoured 
sharks." But from an examination of the microscopic struc- 
ture of their boat-like shields evidence is obtained of their 
affinity with Thelodus and Ateleaspis. The presence of cer- 
tain falciform plates also strongly supports this reference by 
suggesting an affinity to the similar parts, just mentioned, of 
Drepanaspis. On this view the group of "armoured sharks" 
of some writers disappears by their transference to the 
Heterostraci. 
There is less that calls for remark in the chapter on the 
pteraspids. Prof. Traquair, however, takes the view that 
these fishes possessed no fins and that the objects taken for 
such by some writers are of a dififerent nature. His reference 
to the opinions of Jackel and Dean is, however, scarcely con- 
clusive and the fossils cannot be explained in that view with- 
out a forced construction of their meaning. 
It follows, then, that Prof. Traquair maintains the view 
that the Elasmobranchs are the earliest known forms of fish, 
and he would derive the other group of Osteostraci and Hete- 
rostraci from these l\v continuous ossification and consoli- 
dation of the tubercles of Thelodus, or of the other Coelole- 
pidse. In this case we must consider the sharks as the sur- 
viving form and the others as forms which, in consequence 
of lack of harmony witb. the environment, have failed in the 
struggle for existence. At the sanK> time it is scarcely pos- 
sible to doubt, after a study of the various groups of fishes, 
both fossil and recent, that the teleosts have more probably 
