xiv. LOCAL DIVERSITY OF LIFE-FORMS. 409 



pression must be of similitude between them. In each case there 

 is abundance of conchifera, fewer gasteropoda, still fewer cephalo- 

 poda. Plants of terrestrial growth occur in both, and are much 

 alike. 



In the Colly weston slates, among the more common bivalve shells 

 we have of monomyaria 



Avicula, a small species in abundance. 

 Gervillia acuta in great profusion. 

 Pinna cuneata frequent. 



And of dimyaria 



Astarte elegans. 

 Cardium cognatum. 

 Ceromya Bajociana. 

 Modiola imbricata. 

 Modiola Sowerbyana. 

 Myacites sequalis. 

 Trigonia impressa. 

 Unicardium gibbosum. 



These are all frequent, and sometimes very plentiful, 

 with valves in the original apposition, or opened, 

 but not disjoined ; in this respect the resemblance 

 to the Brandsby slate is perfect. 



This list affords no sure ground for joining or separating the 

 zones of Colly weston and Stonesfield ; but may be trusted so far 

 as to confirm the influence obtained from the comparative sections. 

 These slaty rocks may very naturally be regarded as belong- 

 ing to one calcareo-arenaceous group of strata, which, by reason 

 of the absence of fuller 's-earth clays, does really include the lower 

 part of the Great oolite, with some portion not yet clearly deter- 

 termined of the Inferior oolite. And so we leave it for further 

 research f . 



f For much information on the Northampton sands, consult Sharp in Geol. 

 Proc. vol. xxvi. 1870. 



