A DcccMivc Fossilizaticn. — Sardcson. 45 
The variability of hinge in AI. plana H. is not to be taken 
as limited to that which is indicated b_y the few known super- 
ior specimens, but the details of it as given correspond well 
with the great variability of other Ordovician species such as 
Vanuxemia rotunda Hall (numerous synonyms) in which 
no constant munber of teeth can be ascribed to the species, 
although they are few, large and sharply defined, and always 
appear on the fossil, the variation being as great as here found 
in M. plana H. And likewise the presence or absence of hinge 
teeth may be specific only as further suggested by the few,, 
obtuse teeth of rudimentary appearance on M. plana H. 
What use there may be for the generic name Eurymya 
Ulr., if any, is not decided, but the following considerations 
are suggested by the evidence here presented. If as much lati- 
tude be ascribed to the genus in proportion as there are vari- 
able as compared to invariable characters in the type species, 
then Eurymya might readily include or be included in the 
older named genus, Matheria Billiit^^s. Certainly the genus 
Eurymya a-s described by Ulrich (aside from any mistake as 
to hinge structure), has less latitude in characters than that 
now seen in the type species. Eurymya redefined in accord 
with the characters of M. plana as now understood would be 
included in the definition of his later genus Modiolodon (op. 
cit., p. 521.) ; "Ovate shells of the same general type as Mo- 
diolopsis and Modiomorpha, but having from one to three 
oblique cardinal teeth in each valve." This definition seems more 
adequate for a genus which is to include plastic Ordovician 
species, and in absence of means for scientific revision of these 
fossils, there must remain an open question whether this might 
not better be extended to include also less than one hinge tooth, 
referring then all these species back to the genus ^lodiolopsis 
for the present. 
