OSTRACODERMI 523 



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. Xor 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. 1 On these grounds it would seem 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. _Y<//. .SW. xi. 1897. p. -If). 



