Note 5 — INTRODUCTION.

For purposes of comparison other than European frogs were examined, especially American species of Rana, for which I am indebted to the kindness of my much esteemed friend Agassiz. These were Rana Catesbyana, Shaw (R. mugiens, Catesby—R. pipiens, Harlan); R. sylvatica, Leconte; R. clamitans, Daudin; R. palustris, Leconte; R. halecina, Leconte. To these, however, no further reference will be made in the text.