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 

 in 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 8tructure,J)ut 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.] 60 



