26 THE INDIGENOUS FAUNA. 



Now our most troublesome points were in the connexion be- 

 tween Moutoniana, microtrema, and sella; between microtrema 

 and Meyeri ; and between Waldheimia pseudojurensis and W. 

 tamarindus. Now if from the groups of Terehratula and Wald- 

 heimia we select all the species that are known in rocks of older 

 date than our Midland Neocomians, we shall have Ter. Moutoniana, 

 T. depressa and T. sella, Waldheimia tamarindus and W. pseudo- 

 jurensis, all of them occurring in the Middle Neocomian of other 

 districts, and some in the Lower Neocomian. Now these are pre- 

 cisely the species where we have failed to establish a perfect series 

 of connecting links, — where our chain seemed weak or broken \ 

 These species had acquired their special characteristics of form in 

 an earlier period and remained distinct. 



The same was doubtless true of the Terebratellse, for these fall, 

 not into one group but into two, and these two types must have be- 

 come differentiated and fixed before the period of our Brickhill 

 deposit, and so of course remained distinct as separate local main- 

 stocks. 



It is noteworthy that at Up ware, and indeed all other places 

 known to me, the species of Brachiopoda maintain much more dis- 

 tinctness and isolation from one another than at Brickhill. And 

 from this fact one might expect to find that the Brickhill fauna 

 flourished at a slightly earlier date than that at Upware, and 

 acquired diverse developments into well-marked types ; and these 

 spreading outwards from this centre of dispersion to other areas 

 (Upware, Potton, etc.), afterwards maintained their special deve- 

 lopments of form with all the fixity which commonly obtains in 

 specific characters. 



In examining the gradational series arranged on tablets in the 

 Woodwardian Museum it should be borne in mind that these 

 tablets do not pretend to be complete in their every connexion at 

 each point. Indeed it is scarcely possible to arrange a perfect 

 lineal series. The characters to be considered are very numerous 

 in each species, and, in the absence of the ideally perfect con- 

 necting specimen, intermediate in all its points, it must suffice in 



^ It is worth recording that the arrangements here described had been com- 

 pleted and the points of difficulty here noticed were recognised before the idea of 

 their connexion with the earher appearance of certain species had suggested itself 

 to me. 



