The only portions of this fossil which have yet been found, are the abdomen and tail. The abdomen is composed of three joints; the first passes from the side lobes completely over the body, and on its upper surface, near the middle of the back, there is often a minute elevated pimple. The other two, pass obliquely from the lateral lobes, and are interrupted in their course over the body. The tail, or posterior portion, is expanded, something like that of the Isotelus or Dipleura, and has a deep puncture on each side, about half the distance between its terminal border and the last articulation of the abdomen. The lateral lobes are unlike those of any other genus. They form narrow cuneiform appendages to the sides; near the first joint of the abdomen they are crossed transversely by an elevated ridge, from which they gradually taper along the sides of the body, and appear to inosculate in a delicate point at the central border of the tail. The abdominal articulations do not pass over these lobes, but just below the last joint, a little transverse furrow, in perfect specimens, may be noticed. The largest specimen of this fragment I have seen is exactly half an inch in length. This fossil occurs in black shaly limestone, on the canal near Cahooes Falls, in the State of New York, and at a number of other places in that State. I have named this species in compliment to my early friend, Professor Theodore Romeyn Beck, M. D., well known both at home and abroad, as the learned author of the work on Medical Jurisprudence. Some time after commencing this little Monograph, I communicated my plan to Dr. Beck, and was surprised and gratified to find that he was also engaged with the same inquiries, and that he was then busy in arranging and examining the unique collection of trilobites belonging to the Albany Institute. Without the smallest hesitation, he placed all his specimens at my disposal, and has facilitated otherwise my undertaking, by every means in his power. Genus Nuttainia. _Eaton._ Professor A. Eaton, in his Geological Text Book, has proposed the Genus Nuttainia, to include two remarkable trilobites which could not properly be arranged in any of the previously established genera. The two fossils here grouped together, bear no generic relation to each other. The first species which he calls N. Concentrica, belongs to the genus Cryptolythus, which was proposed before the appearance of his work, and has therefore been noticed in another place. The genus Nuttainia is thus characterized by its author: "Head in three lobes, the middle one most prominent; the two lateral lobes sub-hemispherical, or sub-quadrantal; the whole head bordered anteriorly with a punctured fillet; body distinctly three lobed, middle lobe sub-cylindric, and not so broad as the side lobes." Nuttainia Sparsa. _Eaton._ Cast No. 35.