Ill 



Dimensions of valve of Leptobolus (mut.) insulse. From assise E. 2 (a 1 ?). 



Dimensions 

 &c. of this 

 form. 



A comparison of the size and form of Leptobolus in the most typical 

 group, gives interesting results. It shows a development in these Basal 

 Cambrian rocks by an increase of the valves both in length and breadth, 

 which is parallel to the development of Acrotreta in the Cambrian 

 rocks, as a whole, as shown above, and in the writers paper on the Cam- 

 brian Acrotrotidse, etc.* This will be seen by the following table in 

 which only the ventral valves are considered. 



Development of the phylum of Leptobolus atavus in the Basal Cam- 

 brian rocks. 



*Additional notes on the Cambrian of Cape Breton, etc. Bull. Nat. Hist. Soc., N.B., 

 vol. iv, p. 377, etc. 



