BRACinOPODA. 



2S3 



in Leptcena rhomboidalis, persisting tliroughout so m.any successive faunas, forms 

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



The leptsenoid type of structure attained its most abundant exemplification 

 in the fiiunas of the Silurian. Leptczna 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 l.)y a number of species in American faunas, e.g., Leptana fasciata. 

 Hall, of the Chazy; L. allernata, Conrad, L. alternistriata. Hall, L. deltoideu, Con- 

 rad, Strophomena camerata, Conrad, and Leptana incrassata, (Hall) Safford, of tlie 

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

 Billings, Leptczna allernata, Conrad, Strophomena squamula and S. Ulrichi, James, 

 from the Hudson River group. 



In the Clinton fauna the Leptana 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, Sovv- 

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

 to this group, such as Orthis Orbigiii/i, Leptczna Waltoni and Strophomena Hender- 

 soni, Davidson. 



Fiii. 20. Sii-ophomenn [Jin/iiK^'/uinti , Jnh\^i, Davidson. 

 Alter Davidson. 



