52 PTEEOPODA. 



shell, but there is one curious little form (the T. fissurclla 

 of the Devonian), which, if rightly referred to this genus at 

 all, has tlie peculiarity that the shell is destitute of the 

 cliaracteristic rings. This species is further remarkable as 

 occasionally occurring in such extraordinary profusion as 

 to actually form beds of limestone ; as observed by the 

 author and by Mr George Hinde in a thin limestone in 

 the Devonian (Genesee Slates) of North America. 



