When we first noticed the remarkable lunate appearance of the tail of this Asaph, we supposed that it was occasioned by some accident, but there seems no doubt that this conformation is natural. In our specimens of this species, which are not however perfect, the articulations of the abdomen do not exceed 8 in number. The representation of this animal remain given by Mr. Eaton, plate 1, figure 1, is exceedingly inaccurate; it will confuse rather than illustrate the subject. Our cast and the drawing, we believe, are taken from the same specimen, which was kindly loaned by Mr. Eaton for this work. It is but justice to the amiable, industrious, and indefatigable author of the Geological Text Book to remark, that he regrets as much as any one, the insufficiency of his figures of the trilobites, to give any correct idea of the fossils they are intended to represent. In the cabinet of the Albany Institute there are a number of specimens of the A. Selenurus. One of our models represents the natural mould made by the animal in the rock; the other is an impression taken from it, in order to exhibit the animal in a more satisfactory manner. Asaphus Limulurus.[28] _Green._ Cast No. 16. [Footnote 28: From two Greek words, which signify "Limulus tailed."] Cauda longa, spina munita sicut in Limulo; costis abdominis in spinis retrorsum flexis, desinentibus. It is very much to be regretted that the abdomen and caudal end only of this remarkable Asaph have hitherto been discovered; it is, however, exceedingly gratifying that the fragment still remains in so perfect a state. It forms a part of the magnificent cabinet of organic remains belonging to J. P. Wetherill, Esq., now deposited in the Academy of Natural Sciences, of Philadelphia. Dr. J. J. Cohen discovered a small specimen of this species at Lockport, New York, which he has presented to the Athenæum, in Baltimore. Eight articulations of the abdomen, and ten of the tail, are all of this fine species that we have seen. The ribs, or costal arches of the abdomen have a deep furrow on their upper surface, commencing at the middle lobe, and terminating near their free extremities; these extremities appear all detached from each other, and end in reflected points or spines, so as to give the side of the animal a serrate appearance. The costal arches of the tail are grooved through their whole extent, and present no spinous terminations. Beyond the membranaceous expansion of the tail, which is somewhat similar to that of the Asaphus Caudatus, there projects a single spine, like that from the tail of the _Limulus polyphemus_; this spine may be traced under the caudal membrane to its insertion into the middle lobe. A portion of the crustaceous shell is still entire, and it seems to have been covered with very minute granulations. A row of large granulations may easily be traced on each side of the middle lobe. Length of the fragment, one inch and a half. Breadth one inch and a fourth.