GIRTT, GEOLOGIC AGE OF THE BEDFORD SHALE 301 



ing, in proportion to their size, — long as the biologic rank of the group 

 increases, short as the biologic rank of the group decreases. 



Our knowledge of the range of fossils is conditioned by other things 

 than the amount of data as to their occurrence, since it manifestly in- 

 volves the identification of the fossil as a species and the identification of 

 its geologic horizon. Another factor then which makes some forms 

 more significant than others is that their generic and specific relations 

 can be determined with greater certainty. Variation in this particular 

 is in some cases intrinsic, in others extrinsic; often it depends upon 

 both factors. Manifestly, in groups where variation is restricted in 

 degree, or is restricted to a few characters, or is marked by complete 

 intergradation, discrimination of species and even of genera is more 

 difficult than others. Types which vary in shape alone have in my ex- 

 perience proved especially unsatisfactory, while those which are sculp- 

 tured or which possess other features of relief give more reliable results, 

 especially when, as is almost always the case, this is combined with varia- 

 tion in configuration also. Preservation, which in fossils has always 

 destroyed the soft parts and the coloration, often obscures other charac- 

 ters too, and this deterioration, owing to peculiarities inherent in whole 

 groups of shells, is more liable to befall some types than others. As is 

 well known, owing to their physical rather than their chemical structure, 

 the shells of pelecypods and cephalopods are apt to be removed by solu- 

 tion so that in Paleozoic rocks they are as a rule reduced to the condi- 

 tion of molds, while the shells of brachiopods retain their original compo- 

 sition. Also, partly because of the solution of the shell, which tends to 

 obscure both sculptural or specific and structural or generic characters, 

 partly because of being marked by growth lines alone and having the 

 generic characters largely developed along the hinge where they are at 

 best difficult of observation, many types of pelecypods show differences 

 only in shape and configuration (which are peculiarly liable to be altered 

 by compression), while at the same time quite widely different genera 

 are in general aspect, which is about all that can be determined, very 

 similar, — for these reasons pelecypods often prove an unsatisfactory 

 group for stratigraphic paleontology, since even the generic position of 

 specimens as they ordinarily occur in the Paleozoic is often undeter- 

 minable except on characters which are not of themselves strictly generic, 

 although they are, or appear to be, correlated with generic characters. 



It is obvious that, while the larger zoologic groups are as a rule of less 

 value because of their longer range, they are at the same time usually 

 of greater value because of the precision and certainty with which they 

 can be distinguished, for it hardly needs to be stated that a specimen 



