16 



T. 0. BoHWorth— Outlines of Oilfield Geolog;/. 



structure rather than true phylogenetic affinity, tlie tendencies^ of 

 evolution of the apical system may be expressed in the following 

 diagram : — 



' PlesiecJdnus ' 



Anorthopygus 



Pygaster, sens. sir. 



Spatangus Conulus 



Echinohrissus Holectypus, sens. sir. 



Ccenholedypus Disco'idea 



Pyritia, Hemiaster, Molader 



Clypeaster 



EXPLANATION OF PLATE II. 



The fi 

 stated, ar 

 Fig. 1. 



,, 2. 

 3. 



,, 4. 



,, 5. 



,, 6. 



,, 7. 



,, 8. 



,, 9. 



,, 10. 



,, 11. 



,, 12. 



,, 13. 



;ures are not drawn to a definite scale, and, except where otherwise 

 e generalized from several specimens. 

 Apical system of Pygaster {' Plesiechinus ') macrostoma (after 



Wright). 

 Apical system of P. (' Pygaster- ') umbrella. 



,, ,, Anorthopygus michelini (after Cotteau). 



,, ,, A. orbicularis. 



,, ,, Holectypiis (Holectypus) depressus. 



,, ,, H. [Ccenholectypus) cenomanensis (after Cotteau). 



,, ,, Discoidea cylindrica. 



,, ,, Conulus albogaler us. 



,, ,, Pyrina desinoulinsi. 



,, ,, Hemiaster or Micraster. 



,, ,, Spatangus. 



,, ,, Clypeaster. 



,, ,, Holaster or Eclmiocorys. 



Y., — Outlines of Oilfield Geology. 

 By T. 0. BoswoKTH, B.A., B.Sc, F.G.S. 



Introduction. i 



iT is remarkable that in English geological literature and in our 

 Universities so little attention is given to this important branch of 

 geology. Probably it is because in this country petroleum occurs 

 only in small quantity; nevertheless, in many parts of the British 

 Empire petroleum mining has become a great and increasing industry, 

 and is one of the chief directions in which geology is of practical 



