No. 440.] 



CORRESPONDENCE. 



577 



of Pterichthys, Bothriolepis or Asterolepis, we note at once that 

 the contour of the body-wall is inflected, or even indented, on 

 either side at the point where the pectoral limbs peculiar to 

 these forms were attached (Fig. 2). This latter condition being 

 governed by mechanical principles, as anyone can perceive, it is 

 common only amongst those forms which possess swimming 

 appendages. And its absence in Tremataspis and generally 

 throughout the order to which it belongs fortifies us in our con- 

 clusion that in this group paired segmental appendages were 



