LAMELLIBRANCHIATA. 533 



OrytodontldBB.] 



guished by its greater hight, different outline, and finer radiating strise. The latter 

 are also most distinct in that species centrally where they are wanting entirely in 

 P. striatula. 



Formation and tocaWg/.— Middle Galena, Pleasant Grove, Minnesota. 



Family CIRTODONTID^, 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, Ptychddesma, 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 Arcidce have 

 been most favored. The conclusions of the authors seem to have been biased by a 

 supposed resemblance between the hinges of Cyrtodonta dMdi Macrodon and to 

 Stoliczka 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 Area. 



There is something decidedly suggestive in the resemblances to be noted in a 

 comparison of the interiors of true Arcidce 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 Arcidce, since th^it genus 

 most certainly did not arise in Ctenodonta. 



