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K. J. CHAPMAN 



by a ridge or baud with the glabelhi, narrow body-axis with numerous or comparatively 

 numerous segments, and small, short pygidium. Again in the Cambrian Dikeloeephalus, 

 and in Cteuopyge (Linuarsson), the pygidium is very large — thus presenting a marked 

 departure from the so-called primordial type. Other examples might be cited to prove, 

 (1) that these imaginary primordial characteristics are present in various post-primordial 

 genera, and (2) that they are not always present in primordial or Cambrian tyi^es. 



(4.) Nothing. p<^rhaps, shows more forcibly the arbitrary, unnatural character of 

 stratigraiihical groupings, than the collocation in recent classiiications of Neseuretus, 

 Hicks, side by side with Paradoxides in the family of th<' Olenidte. In all its leading 

 characters, Neseiiret as is simply a Cambrian Calymenc, probably the ancestral source of 

 the latter type ; l)ut in the classifications referred to, these genera are placed in ditlerent 

 families and widely apart. To show these points in all their distinctness, viz., the close 

 agreement of Neseuretus with Calymeue, and its remote relations to Paradoxides, a com- 

 parative view of their more characteristic structures is given in the following table ; — 



In these recent clajsiiicatious, also, we find Dikelocephalus arranged under the 

 Olenida*, whilst Arethusina and Harpides, which agree very strikingly with Olenus in 

 their more salient characters, are placed far apart from the latter under the Proetidte, 

 evidently upon purely stratigraphical grounds. The structural relations oi these genera 

 are brieflv indicated in the annexed tabular view : — 



