BRA CHI OP ODA . 
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in Leptma 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. Leptma 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., Leptaina fasciata, 
Hall, of the Chazy; L. alternata, Conrad, L. alternistriata, Hall, L. deltoidea, Con¬ 
rad, Strophomena camerata, Conrad, and Leptma incrassata, (Hall) SafFord, of the 
Trenton; S. Mimesotensis, N. H. Winchell, S. Kingi, Whitfield, S. nitens, 
Billings, Leptma alternata, Conrad, Strophomena squamula and S. Ulrichi, James, 
from the Hudson River group. 
In the Clinton fauna the Leptma 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 ( Leptma ( 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, Leptma Waltoni and Strophomena Hender- 
soni, Davidson. 
Fig. 20. Strophomena ( Rafinesquina) Jukesi, Davidson. 
After Davidson. 
