LAMELLIBRANCHIATA. 533 
Orytodontidz.] 
guished by its greater hight, different outline, and finer radiating strie. The latter 
are also most distinct in that species centrally where they are wanting entirely in 
P. striatula. 
Formation and locality.—Middle Galena, Pleasant Grove, Minnesota. 
Family CYRTODONTID A, n. fam. 
Shells commonly ovate or rounded, rarely elongate, valves generally ventricose 
or strongly convex. Shell substance calcareous, without epidermis, usually thick. 
Hinge plate often massive, strong, with from one to five cardinal teeth; elongate 
posterior lateral teeth usually present, but may be wanting. Ligament chiefly 
external. Anterior adductor scar strongly impressed, rather large though always 
smaller than the much more faintly impressed posterior adductor. Pallial line 
simple. 
The genera included in this family seem to form a very natural group. With 
one exception, Ptychodesma, Hall, a Devonian genus, they are all restricted to the 
Lower and Upper Silurian rocks and many of the species rank among the most 
important fossils of the various beds in which they occur. The individuals also are 
often very abundant, while their preservation is on an average better than that of 
any other group of paleozoic bivalves. 
The principal genera are variously placed by systematists, but the Arcide have 
been most favored. The conclusions of the authors seem to have been biased by a 
supposed resemblance between the hinges of Cyrtodonta and Macrodon and to 
Stoliezka the relation is so obvious that he is led to say “the former may be consid- 
ered as the predecessor of the latter in geological history.” Now, after careful 
examination, I am obliged to dissent in so for at least as to claim that the case is 
far from proved. So far as we can now tell the last species of Cytodonta (Upper 
Silurian) are as far removed from Macrodon as are the earliest, while the first 
species of Macrodon (Devonian) is no nearer Cyrtodonta, than are the Jurassic forms. 
Even should later discoveries prove a development of the latter from the Silurian 
genera under consideration, it would not settle the question for it is not by any 
means an established fact that Macrodon is genetically related to Arca. 
There is something decidedly suggestive in the resemblances to be noted in a 
comparison of the interiors of true Arcide like those of the genus Barbatia, Gray, 
and certain species of Ctenodonta, Salter. Now if these should, as I am inclined to 
believe, indicate something more than a merely accidental agreement of structure, 
I should hold that Macrodon was not a member of the Arcide, since that genus 
most certainly did not arise in Ctenodonta. 
w 
