the indigenous fauna. 

 Genus Terebratula. 



Terebratula Cantabrigiensis (Walker) 



♦ cyrta (Walker) 



«>S 



depressa 



Seeleyi (Walker) 



•uniplicata (Walker) 



prselonga (Sow.) 



( ►Dallasii (Walker) 



( "-Lankesteri (Walker 



microtrema (Walker) 



[ *extensa (Meyer) 



{ Meyeri (Walker) 



Moutoniana (d'Arch.) 

 sella (Sow.) 



1 

 Tornasensis (d'Arch.) 



,, Upwarensis (Walker). 



Explanation. The arrows indicate the lines of variations and probable de- 

 velopment. The dotted lines signify less perfect continuity and connexion. 



This series may be studied in the drawers of the University 

 collection, Cambridge. 



Amongst all these species the connecting morphological varie- 

 ties are by no means equally evident or of equally frequent occur- 

 rence. Between prcelonga and microtrema the connecting varieties 

 are very abundant, so that in any dozen specimens, casually taken, 

 one is pretty certain to meet with the difficulties of separation ; 

 and indeed through all the linear series in the plan from T. Can- 

 tabrigiensis to T. microtrema the passage is simple and clear. 

 Again, the varietal offshoots from depressa {cyrta and uniplicata), 

 and the lateral branches from prcelonga, known as Lankesteri and 

 Dallasii, are satisfactorily connected with the main stock. But 

 much more searching and much greater care is required to estab- 

 lish the series between T. Meyeri and T. Moutoniana; nor is the 

 result so satisfactory as in the other cases; it is in fact remarkable 

 that it proved to be much more difficult to arrange a series between 

 these two somewhat similar shaped shells than between the far 



