ore OSTRACODERMI 52 
Ww 
Of the three subordinate groups or “ Orders ” into which they have 
usually been divided hitherto, two, the Heterostraci and the Osteo- 
straci, may, with some show of reason, be considered as related 
forms, and although they are characterised by much specialisation 
on independent lines, there is yet some evidence of connecting links 
between the two. The organisms comprising the third group, 
the Antiarchi, stand upon a very different footing, and at present 
it cannot be said that they are in any way related to either 
the Heterostraci or the Osteostraci, or indeed to any other 
Craniates whatsoever. The association of the Ostracodermi with 
the Cyclostomata, a view which has received more influential 
support than it deserves, is based on the presumed absence of 
jaws and paired fins. The absence of jaws, which, if present, 
were almost certainly cartilaginous, has yet to be proved, and 
even in the latter group it is by no means certain that they do 
not possess structures which, morphologically if not functionally, 
are veritable jaws. Nor is it quite certain that the lateral 
lobes of some Ostracodermi are neither pectoral flaps nor lateral 
fin-folds, to say nothing of the lateral appendages of the 
Antiarchi. And to these objections there is the further difficulty 
that there is absolutely no evidence that the Ostracodermi are 
monorhinal in the sense in which this term is applied to the 
Cyclostomata.’ On these grounds it would seen: more in 
accordance with our present knowledge to regard the Ostra- 
codermi as an independent group whose exact position in the 
system has yet to be determined, including, however, besides the 
generally accepted orders Heterostraci and Osteostraci, the recently 
founded provisional order Anaspida, but excluding the Antiarchi 
as a separate and distinct section; rather than to crystallise in 
a definite system of classification views which are either purely 
conjectural or wholly unjustifiable. Even with this limitation 
the Ostracodermi are by no means easy to define, especially if 
we include those remarkable shark-like forms from the Upper 
Silurian rocks of the south of Scotland which have been so 
admirably described in the recent classical memoirs of Dr. 
Traquair. As a rule, the head and the anterior part of the 
body are laterally expanded, and more or less sharply defined 
from the rest of the body by prominent postero-lateral angles. 
The exoskeleton, which exhibits an extraordinary variety of 
1 Lankester, Nat. Sed. xi. 1897, p. 45. 
