1882.] 



ome those common to both, 

 i and characteristic species : 



As will be seen there are three species which have been con- 

 sidered to pass out of existence with the Clinton, that still survive 

 in the stratum under consideration ; unless we should also include 

 the Leptaena described below. 



Mtnstina intermedia has been placed in the list of species com- 

 mon to both groups, as the one from the Transition stratum, as 

 well as those from the Clinton, are indistinguishable from speci- 

 mens found in the Niagara shale; the only perceptible differ- 

 ence being a gradual falling off in size as we advance upwards in 

 the series, and a slight diminution of the average width, so that 

 some from the Niagara approach closely to the typical Jf. nitida. 

 although others will be found that are fully as wide as those from 

 the Clinton. 



In all probability M. nitida and M. oblaia are descendants of 



