The earliest discovery of trees of this kind in the Erian of America was that of Matthew and Hartt, who found large trunks, which I afterwards described as _Dadoxylon Ouangondianum_, in the Erian sandstone of St. John, New Brunswick, hence named by those geologists the "Dadoxylon sandstone." A little later, similar wood was found by Prof. Hall and Prof. Newberry in the Hamilton group of New York and Ohio, and the allied wood of the genus _Ormoxylon_ was obtained by Prof. Hall in the Portage group of the former State. These woods proved to be specifically distinct from that of St. John, and were named by me _D. Halli_, _D. Newberryi_, and _Ormoxylon Erianum_. The three species of _Dadoxylon_ agreed in having composite medullary rays, and would thus belong to the group _Palæoxylon_ of Brongniart. In the case of _Ormoxylon_ this character could not be very distinctly ascertained, but the medullary rays appeared to be simple. I am indebted to Prof. J. M. Clarke, of Amherst College, Massachusetts, for some well-preserved specimens of another species from the Genesee shale of Canandaigua, New York. They show small steins or branches, with a cellular pith surrounded with wood of coniferous type, showing two to three rows of slit-formed, bordered pores in hexagonal borders. The medullary sheath consists of pseudo-scalariform and reticulated fibres; but the most remarkable feature of this wood is the structure of the medullary rays, which are very frequent, but short and simple, sometimes having as few as four cells superimposed. This is a character not before observed in coniferous trees of so great age, and allies this Middle Erian form with some Carboniferous woods which have been supposed to belong to _Cordaites_ or _Sigillaria_. In any case this structure is new, and I have named the species _Dadoxylon Clarkii_, after its discoverer. The specimens occur, according to Prof. Clarke, in a calcareous layer which is filled with the minute shells of _Styliola fissurella_ of Hall, believed to be a Pteropod; and containing also shells of _Goniatites_ and _Gyroceras_. The stems found are only a few inches in diameter, but may be branches of larger trees. It thus appears that we already know five species of Coniferous trees of the genus _Dadoxylon_ in the Middle Erian of America, an interesting confirmation of the facts otherwise known as to the great richness and variety of this ancient flora. The late Prof. Goeppert informed me that he had recognised similar wood in the Devonian of Germany, and there can be no doubt that the fossil wood discovered by Hugh Miller in the Old Red Sandstone of Scotland, and described by Salter and McNab, is of similar character, and probably belongs to the genus _Dadoxylon_. Thus this type of Coniferous tree seems to have been as well established and differentiated into species in the Middle Devonian as in the succeeding Carboniferous.