[Footnote 38: The general usage of naturalists is to prefix a short Latin caption to the species which they discover--but as some authors do not follow this fashion, we are satisfied to suffer their descriptions to stand without it. We believe, indeed, that the time is not very distant, when every author will be expected to publish his discoveries in his vernacular tongue.] _Head_ more rounded than the preceding, and less elevated. _Tail_ flat, rounded. Total length two inches and one-tenth. Breadth one inch and one-tenth. Length of the head, six-tenths--of the abdomen, eight-tenths, and of the tail seven-tenths. Dr. Dekay is of opinion that this species may possibly prove to be the young of the preceding. The relative proportions of its buckler and tail vary considerably from those of the I. gigas; and the depth of the lateral lobes, which exceeds three-tenths of an inch, would almost of itself determine it to be a new species. The original, from which our cast was taken, is in the cabinet of J. P. Wetherill. It was found near Newport, Kentucky, and occurs in argillaceous slate. The fossil is of a dirty yellow colour. Isotelus Cyclops.[39] _Green._ Cast No. 24. Fig. 7. [Footnote 39: From the Greek for "round eyes,"] Clypeo antice attenuato, plano; oculis rotundis, proximis; cauda ovata, acuminata. The head of this species is much more elongated than it is in the two preceding species. The anterior portion of the buckler is much prolonged. The eyes are approximate, rounded, and near the posterior edge of the head. The abdomen is furnished with eight distinct articulations; the middle lobe is scarcely broader than the lateral lobes; tail rather broader than the head, and ovate; posterior termination more rounded than the buckler. Length nearly three inches. The specimen from which our model was taken belongs to the Albany Museum. No label is attached to it, but I was informed by Mr. Meach, one of the proprietors, that it was found in the western part of the State of New York. It is embedded in an ash-coloured limestone. The specimen is a good deal worn--but the peculiar form of the eyes, and the narrowness of the middle abdominal lobe, clearly distinguish it from either of Dr. Dekay's species. Isotelus Megalops.[40] _Green._ Cast No. 25. [Footnote 40: From the Greek for "great eyes."] Clypeo antice subrotundato, postice arcuato; oculis magnis, rotundis, eminentissimis; cauda suborbiculari, limbo lato; articulis abdominis octo.