Asaphus Pleuroptyx.[33] Cast No. 18. [Footnote 33: From the Greek word for "grooved ribs."] Corpore depresso; cute coriacea tuberculis minimis; costis striatis; cauda acuta, brevi. This species like most other specimens of this genus, in our cabinets, is decapitated--every other part, however, appears to be in a good state of preservation. The articulations of the abdomen and tail, which cannot readily be distinguished from each other, are seventeen in number. The middle lobe is flat, and regularly tapers to an obtuse lip; it is marked on each side with longitudinal impressed lines or little grooves. The costal arches on their upper side have a deep and narrow channel, running through their whole course. The costal arches of the abdomen have no membranaceous expansion beyond their terminations; this organization is only visible immediately below the end of the middle lobe, where it quickly finishes in an acute point. A large portion of the crustaceous shell remains, and is covered with distinct granulations; those on the tail are the least obvious. This species approaches very near the A. caudatus, but the grooves on the middle lobe, the smallness of the costal arches, and the limited extent of the membranaceous expansion round the lower portions of the shell, will sufficiently distinguish it. Two specimens of this Asaph are in the cabinet of the Albany Institute. The one from which our cast is taken, was found on the Helderberg mountains; it is embedded in a light grey coloured limestone shale. The other specimen, which is much smaller, was discovered near the Genessee River, in the State of New York. The rock in which it occurs is identical in its constitution with the other. It contains other species of trilobites, and a number of shells. Asaphus Micrurus.[34] Cast No. 19. Fig. 3. [Footnote 34: From the Greek, for "minute tail."] Cauda attenuata, acuta; corpore valde convexo; costis striatis; parte marginali vix membranacea. This fine, large caudal termination of an Asaph is in the cabinet of the Albany Institute--and it is a subject of great regret, that all that has yet been discovered relating to this highly interesting trilobite, is to be seen in this fragment. There are eighteen articulations of the tail and abdomen, which cannot be distinguished from each other. The middle lobe is composed of a series of straight, distinct, parallel articulations, very convex about the middle, so as to form a kind of longitudinal ridge down the back. The costal arches of the lateral lobes are very distinct, and are longitudinally striated or grooved on their upper surface, particularly those near the upper part of the animal. The membranaceous expansion is very narrow along the sides of the body, and forms a sort of hem; below the central portion of the tail it makes a short acute projection, which seems to be supported by a short costal elongation of the middle lobe. Length two inches and a half.