Strophomena and Other Fossils 
127 
It is doubtful whether it will be possible to make use of these 
distinctions between the Richmond and the Eden forms of Plec- 
tamhonites except locally. The variations are of the lower rank 
to which the term mutations rather than varieties, has been 
applied recently. The term Plectambonites rugosa-clarksvillensis 
will serve to designate the large specimens from the lower third 
of the Clarkvsille division of the Waynesville bed, in Ohio and 
Indiana. With these specimens, those from other horizons in the 
Clarksville and Blanchester divisions of the Waynesville, and 
those from the Liberty bed, are regarded as identical. 
The difference in outline from transversely elongate quadran- 
gular to semi-elliptical frequently is shown with all intermediate 
stages on the same slab. All of the shells belonging to the Plec- 
tamhonites rugosa group increase more in width than in length, 
when attaining a large size. The resultant outline is a matter 
of luxuriant environment rather than of specific change, and is 
shown more conspicuously by the larger specimens than by the 
smaller specimens. 
It will be scarcely worth mentioning that these slight differ- 
ences between forms found at different horizons will be found of 
interest more by the stratigraphical paleontologist, than by the 
student of biology. However, if such species as Plectambonites 
sericea are to be of any value for the closer correlation of the 
smaller divisions of Ordovician formations, the attempt must be 
made to discriminate between the smaller variations. Whether 
the present attempt has added anything or not, is another ques- 
tion. 
Rafinesquina declivis, James 
{Plate VIII, Fig. 10, enlarged) 
An enlarged drawing of the same type specimen as that repre- 
sented by Fig. 12 C, on plate V, in vol. XVI of this Bulletin, 
is here presented, in order to give a better idea of the alternation 
of single stronger striae with sets of two or three finer striae. 
The median striation is the most prominent. 
