Some years ago the British Museum acquired from Dr. William Clark a collection of the remarkable Arthrodira found by him in the Upper Devonian shales of Ohio. In this material there are several specimens which were described as new before being brought to Europe, but concerning which there has been more or less doubt among students of the Arthrodira. Hitherto, however, it has been quite impossible to dispel this doubt since the material necessary to a comparative study of the forms in question is to be found only in America. Last year I enjoyed the privilege of going over this material in the British Museum, thanks to the courtesies extended to me by Dr. Arthur Smith Woodward, F.R.S., the Keeper of Geology, and as I had just come from a study of the American forms I was in a position to resolve these doubtful cases. The results of this study are presented in the following pages.