.,.,, -1:1.1.11. KAN, IIIATA -,.;.; 



j:ui*lied l,y its greater hight, dillerent outline, an, I liner radiatm- The latter 



are also most distinct in that species centrally win-re they are V entirely in 



trfctofe 



formation and tooaMjf.-Mltldle Galena. Plevna t Grove, Minnesota. 



.ally Cl'RTODONTIDyE, n. fam. 



Shells commonly ovate or rounded, rurely 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 chielly. 

 external. Anterior adductor scar strongly impressed, rather large though always 

 Miialler than the much more faintly impressed posterior adductor. Pallial line 

 simple. 



The generu included in this family seem to form a very natural group. With 

 one exception. /V/rWrv///./. 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 fo--iU of the various beds in which they occur. The individuals also are 

 often very abundant, while their pn->c-r\ation is on an average better than that of 

 any other group of paleozoic l.ivai 



The principal genera are variously placed by systematists, but the An-l,l,t 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 

 Stolic/ka 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 



; in. it K.D, I am obliged to dissent in so for at least as to claim that the case is 

 tar from proved. So far as we can now tell the last species of Cyfalonla (Upper 

 Silurian) are as far removed from Mm radon as are the earliest, while the first 

 species of Mm-rodon (Devonian) is no nearer Cyrtoinnt<i. than are the Jurassic forms. 

 Even should later discoveries prove a development of the latter from the Silm 

 genera under con-ideration, it would not settle the question fur it i* not by any 

 means an established fact that Macro fn is genetically related to I 



There is something decidedly suggestive in the resemblances to be noted in .1 

 comparison of the interiors of tn; / like those of the ge: ray, 



ami certain species of Ctfnodonl<i. Salter. Now if these should, as I am inclim-d to 

 l>elieve, indicate something more than a merely accidental agre* 

 I should hold that Macrodon was not a member of the I- ice that ^ei in- 



most certainly did not arise in ' 'f>-nodonta. 



