GENUS CRYPTONELLA. 
393 
and T. planirostra, species described in a preceding report. Having ascer¬ 
tained that the internal loop in Centronella glans-fagea is essentially simi¬ 
lar to that of C. julia (Winchell), a species having the external form of 
Cryptonella, I supposed it possible that the two genera might be merged 
into one. Since that time, I am not aware that any farther investigations 
have been made, tending to throw light upon this subject. 
It has therefore appeared to me very desirable to learn, if possible, 
the internal structure of any one of those species which I originally 
referred to the Genus Cryptonella. Of two of these, C. rectirostra and 
C. planirostra, I have fortunately been able to obtain an exposition of 
the form and structure of the loop, as will be shown in the illustrations 
of the species. 
The crura have the general form of those in Waldheimia, extending 
\n a long recurved loop, with long processes descending into the ventral 
valve, between which and the apex the crura are united by a trans¬ 
verse band, differing in this respect from that genus, while the muscular 
impression and .extended beak offer other distinctions. 
I have thought it desirable, therefore, to continue the name Crypto¬ 
nella to include these two authentic species; while the other species 
are included under it from their general form. 
I have continued the Terebratula lens under that genus, without knowing 
its internal structure, but from its external similarity with those which 
have proved to belong to that genus. 
These investigations have clearly shown how difficult, or even impos¬ 
sible, it is to refer to their proper generic relations these fossils from 
external form and character alone. 
[ Paleontology IV.] 
50 
