Strophomena and Other Fossils 
123 
Plectambonites rugosa, Meek^^ 
{Plate I, Figs. 7 A, B, C; Plate X, Figs. 7 A, B, C, D) 
The name rugosa, first proposed by James in a list of fossils 
from the Cincinnati Group, in 1871, was later changed by James 
to aspera, in the belief that the name rugosa was preoccupied by 
Leptaena rugosa, a view no longer tenable now that the generic 
name Plectambonites has been adopted for the species typified by 
Plectambonites sericea. 
The types of Plectambonites rugosa were found about 150 feet 
above low-water mark of the Ohio River, at Cincinnati, Ohio. 
This would place them in the Southgate division of the Eden. 
A series of specimens, labelled as the types of Leptaena aspera, in 
the James collection, preserved in the Walker Museum, at Chi- 
cago University, is numbered 1085. None of this series shows the 
interior surface of either valve. The term rugosa was given not 
on account of the oblique wrinkles along the hinge-line but on 
account of the roughened surface of the general exterior surface 
of the valves, especially anteriorly. This roughened surface 
appears due to the presence of numerous very thin overlapping 
films of shell material. These films appear to consist of the same 
extremely fine, silky, fibrous material as that forming the compact 
body of the valves. Sometimes they are traversed by the same 
radiating striae as those seen on that part of the exterior surface 
of the valves where the films are not present. The films may be 
more or less discrete from one another, but in some specimens they 
are built up into a solid mass, resulting in a thickening of the 
valves exteriorly. At the exterior margin of the pedicel valve, 
this thickening may reach a total of fully 2 mm., and frequently 
the anterior, more or less vertical slope of this thickening is 
crossed by lines evidently corresponding to the extensions of the 
radiating striae. The thickening usually is confined to the 
anterior half or third of the valves. It may result in a succes- 
sion of concentric bands, the one nearest the anterior margin being 
the most conspicuous. At other times, the thickening increases 
evenly, without any concentric banding, but, most frequently, 
it is more or less irregular, the films being more or less warped or 
^'^Plectambonites rugosa, Ohio Paleontology, vol, i, p. 72; plate V, Figs. 3/, g, h; 
1873. Plecta7nbonites aspera, James, Cincinnati Quarterly Journal of Science, 
p. 151, 1874. 
