The A. Laticostatus occurs in a light coloured ferruginous sandstone, which contains a multitude of other fossil remains, particularly a large species of Productus and of Terebratula. It is said to have been found in Ulster county, in the State of New York, by the late Charles Wilson Peale, Esq., the distinguished founder of the Philadelphia Museum. During the memorable search after the bones of the _Mastodon Giganteum_, in the marl pits of that county, this enterprising naturalist procured our Asaph with many other remarkable petrifactions. The rocks which contain them were probably found not _in situ_, but were masses rolled from the neighbouring Shawangunk mountains,[26] which by some geologists are supposed to be a link in the grand chain of the Alleghanies. Mr. R. Peale, of New York, lately visited the rich repository of fossils in Ulster County, and procured a number of specimens of the A. Laticostatus, all of which he has kindly permitted me to examine. These are much smaller than our cast, but in many instances the caudal elongation is perfectly developed. The A. Laticostatus also occurs in the Helderberg mountains, specimens of which are in the Albany Institute. [Footnote 26: The Lenape tribe of Indians, who formerly inhabited this district of country, gave the name of Shawangunk to this stupendous ridge of hills a name which has been very properly preserved.] Asaphus Selenurus.[27] _Eaton._ Casts Nos. 14 & 15. [Footnote 27: Derived from _Selene_, moon, and _ouros_, tail.] Cauda semilunari; costis angustis, valde distinctis; abdominis articulis duodecim; corpore convexo. I am indebted to Professor Eaton, for two specimens of this very interesting species. In his Geological Text Book, he thus describes it: "Tail crescent-form, or concavo-convex, with the convex side forward, upon which the post abdomen terminates: abdomen contains about 12 articulations, with an abrupt termination equal in breadth to one-fourth of the length of the transverse lunate tail; the articulations of the side lobes gradually incline towards the axis of the body, until the last pair terminate at the tail. Found in transition limestone at Glenn's Falls, and Becroft's mountain, near Hudson. I have a specimen from Becroft's mountain, with part of the original covering of the animal remaining."