On the Structure of the American Pteraspidian, Palæaspis (Claypole); with Remarks on the Family

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Abstract
I. Review of the Discovery of Palæaspis. In the volume of this Journal for 1885 1 the present writer published a description of the first specimens of Pteraspidian fishes that had come to light in the New World. They were discovered in the autumn of 1883 in a bed of variegated marl, and later in a bed of red and green sandstone enclosed in the marl and belonging to the Onondaga or Salina Group of the Upper Silurian formation. The fossils were found in Perry Co., Pennsylvania, in the survey of which the writer was at that time engaged. The sandstone and shale pass under the town of New Bloomfield, and for this reason the stratum has received the name of the ‘Bloomfield Sandstone.’ The horizon of these beds corresponds with that of the English Lower Ludlow, or with the interval between that and the Wenlock, inasmuch as the Lower Helderberg Limestone which overlies it is unquestionably the pal—ontological equivalent of the English Ludlow, and the Niagara Limestone which lies next beneath it is as certainly the counterpart of the: English Wenlock. The whole mass of the Salina Shale is, in Perry Co., about 1500 feet in thickness, and for the most part, with the exception now noted, is entirely unfossiliferous. This mass is but slightly represented in the English series. The fossils consisted of the usual shields resembling those so long known from the rocks of nearly corresponding age in Herefordshire, and occurred in ‘great abundance, but were for

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