part 2] 



ITS STRUCTURE AND ROCK-SUCCESSIOlSr. 



165 



Notes on StropJiomena siliiriana Davidson and 

 Ortliis liirnantensis M'Coy. 



In the Hirnant Beds of the typical locality there occur two 

 brachiopods about which there seems to have been considerable 

 confusion. The following notes are given as a contribution towards 

 clearing up the differences and resemblances which exist between 

 them. The shells are indistinguishable in fragments, for the 



Fig. 10. — Diagrammatic sTcetcTies illustrating differences in typi- 

 cal forms of Orthis hirnantensis M^Coy and Strophomena 

 siluriana Davidson. 



.Cardinal Process 



a 

 X2. 



C.P. 



C.P. 



a = Characteristic appearance of brachial valve in 0. liirnantensis. 

 b & c = Characteristic appearance of brachial valve in two types (alate and 

 non-alate) of Strophomena siluriana. 



ornamentation of the shell appears to be almost identical in both 

 forms, although with a tendency to a greater coarseness in the 

 form here referred to Strophomena. 



