No. 436.] 



APPENDAGES OF TREMATASPIS. 



in mind in view of the great range in structure and function 

 presented by such organs in other animals whose structure is 

 better known than that of Tremataspis, and also in view of the 

 fact that there is often such a great range of variation in a series 

 of these organs in the same animal. Nevertheless in view of the 

 above named consideration and while well aware of the uncer- 

 tainties involved, it seems to me we must not lose sight of the 

 possibility that there may have been more than one pair of 

 jointed appendages in Tremataspis, similar to those described as 

 pectorals, and that the fragment in question may have belonged 

 to one of these additional appendages. 



In conclusion, therefore, I believe that we are justified in the 

 statement that Tremataspis is provided with a pair of oar-like 

 swimming appendages similar to those of Bothriolepis and Pteri- 

 chthys, and that similar appendages were probably present in 

 Pteraspis, Cyathaspis and Tolypaspis. 



In Tremataspis, these appendages were probably attached to 

 the largest pair of incisions on the anterior ventral margin of 

 the head. Appendages attached there would agree (1) in point 

 of attachment with those in Bothriolepis and Pterichth vs. (2) 

 they would be attached to a part of the head best fitted, so far 

 the shape of the head and the location of its centre of gravity 

 is concerned, to serve as the support for a pair of swimming 

 appendages. (3) There is no other part of the head to which 

 such appendages could be attached. (4) They could not have 

 been attached to the trunk, because the trunk of Tremataspis 

 must have been of relatively small size, judging from the way 

 the shield tapers posteriorly, from the small opening at the 

 posterior end, and from what we know of the size of the trunk 

 scales and of the size of the trunk in Thyestis and other Ostra- 

 coderms. It is therefore extremely improbable that large 

 appendages could have been attached to a body so small that 

 the centre of gravity must have been situated in the head, some 

 distance in front of the anterior end of the trunk. 



That by far' the greater share of the work of locomotion in the 

 Ostracoderms must have been performed by the oar-like cephalic 

 appendages is indicated by their anterior position and great size 



