NORTH AMERICAN GRAPTOLITES BGA 
an organ which from its position and character must be regarded as homolo- 
gous with Ruedemann’s ‘central disk” occurs. But in these species the 
“disk” is almost certainly simple, as it has a perfectly definite, uniform 
and constant outline. The compound form cannot then be taken as a family 
character. It may, however, prove of generic rank, Dzplograpsis perhaps 
being distinguished by a compound, and C/imacograptus by a simple disk. 
While it could be conceived as not even having this value, it seems to me not 
improbable that it may ultimately acquire this significance. 
Re Re GURien Van Vieio): 
U. S. GEOLOGICAL SURVEY. 
