Variation Exhibited by a Carbonic Gasteropod. 331 
clayey shales at Crawfordsville, Indiana, has eliminated many 
of the inherent difficulties just mentioned. On account of its 
extensive numerical representation, and also for reason of the 
occurrence of many examples still adhering, as during life, to 
the vaults of crinoids, this species is especially well adapted 
for elucidating certain hitherto obscure points in the import¬ 
ant problem of specific variation. Platyceras equilaterum as 
it occurs at Crawfordsville is here graphically represented by 
the projection of ten specimens (figure 1); eight of which, pro¬ 
jected from above, are also shown in figure 2. In both figures 
the numbers refer to the same individuals respectively ; and in 
all the anterior margins coincide. 
