48 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.54. 



In the present paper, the description of the representatives of the 

 genus Nuculites, a.s expressed in the Silurian formations of Washington 

 County, was made the chief purpose of the investigation. In this 

 study the object in view has not been paleontological but zoological 

 (or rather conchological) , namely, the determination of the proper 

 zoological categories to which the several specimens under investi- 

 gation belong. 



The first point to determine was the meaning of the generic cate- 

 gory Nuculites and the validity of its name. 



Having decided this point, all specimens from the whole collection, 

 which belonged within this category were examined and their mor- 

 phologic characters closely studied. They were classified, labelled 

 and named strictly on the basis of their morphology. The reason 

 for being strict in the apphcation of this rule was the realization that 

 there are several quite diverse causes both for difference and for hke- 

 ness of morphologic characters, neglecting which must necessarily 

 lead to a misinterpretation of the significance of the fossils. 



Some of these diverse causes may be mentioned as self-evident: 



1. The shells of the same zoological species may differ by reason 

 of a natural variabihty in development of the shell in normal growth. 



2. Morphologic differences may arise in addition to natural or 

 inherent variability by reason of differences in food or in conditions 

 of environment. Such a cause is hkely to show itseK on comparing 

 specimens of the same species from distinct localities. 



3. Differences may arise in fossil species from difference in the kind 

 of sediment in which they are imbedded, due to chemical or purely 

 physical causes incident to the solidifying of the rock. 



4. Differences in fossil shells may be caused by ipovements of the 

 rock magma after fossilization. Such changes may be considerable 

 and of unknown amount and without leaving any indication upon 

 the sheU itself of such metamorphism. 



5. Another cause may find its expression in the literature and 

 figures by which Imowledge of fossils is recorded and communicated. 

 The author may associate as characteristics of the species characters 

 observed on separate specimens which he imagines were originally 

 the same species. This results in producing a composite idea of the 

 species, the composition being made up in the author's mind, the 

 reasons for which may or may not be manifest to the reader of the 

 literature or the student of the fossil specimens. 



From these considerations it becomes evident that the scientific 

 record of carefully made observations may be affected by the relative 

 importance the author (it may be quite unconsciously) assigns to 

 one or other of these various causes of the morphologic differences 

 he observes. 



The fossils described in this paper offer such an admirable example 

 of these diverse causes of difference that it has seemed to the author 



