BEACHIOPODA. 



283 



in Leptmna rhomboidalis, persisting throughout so many successive faunas, forms 

 one of the substantial reasons for the separation of that form from its allies. 



The leptsenoid type of structure attained its most abundant exemplification 

 in the faunas of the Silurian. Leptana rhomboidalis, which appeared in the 

 Trenton and Caradoc has alone carried this type onward through the Upper 

 Silurian, the Devonian and into the Lower Carboniferous. Rafinesquina is 

 represented by a number of species in American faunas, e.g., Leptana fasciata, 

 Hall, of the Chazy; L. altemata, Conrad, L. alternistriata, Hall, L. deltoidea, Con- 

 rad, Strophomena camerata, Conrad, and Leptmna incrassata, (Hall) Safford, of the 

 Trenton ; S. Minnesotensis, N. H. Winchell, S. Kingi, Whitfield, S. nitens, 

 Billings, J^eptana altemata, Conrad, Strophomena squamula and S. Ulrichi, James, 

 from the Hudson River group. 



In the Clinton fauna the Leptczna obscura, Hall, may belong to this group, and 

 if so is probably its latest representative in our faunas. In the Silurians of 

 Great Britain the best developed representation of these fossils is in the Caradoc 

 fauna {Leptczna {Leptagonia) ungula, McCoy, L. deltoidea, Conrad, Strophomena 

 Jukesi, Davidson, S. Holli, Davidson, S. siluriana, Davidson, Orthis expansa, Sow- 

 erby). There are a number of species in the Wenlock which appear to belong 

 to this group, such as Orthis Orbignyi, Leptana Waltoni and Strophomena Hender- 

 soni, Davidson. 



Fig. 20. Strophonicna {Jia/uiesquuut) Jukasi^ Davidsou. 

 Alter Davidson. 



