BRACHIOPODA. 



2S3 



in Lept(E?ia rhomboidalis, persisting throughout so many successive faunas, forms 

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



The leptaenoid type of structure attained its most abundant exemplification 

 in the faunas of the Silurian. Lepi(ena 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. allernata, Conrad, L. alternistriata. Hall, L. deltoidea, Con- 

 rad, Strophomena camerata, Conrad, and Leptcuna incrassala, (Hall) SaflFord, of the 

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

 Billings, Leptoina allernata, Conrad, Slrophomena 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 hiunas. In the Silurians of 

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

 fauna {Leplmna (Leplagonia) ungula, McCoy, L. deltoidea, Conrad, Strophomena 

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

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

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

 soni, Davidson. 



Fic^. 20. Strophomena {Iliifincsquina) Jukesi, Davidson. 

 After Davidson. 



