half an inch on its two narrower ones.[AQ] [Footnote AQ: This bone has been since assigned by Agassiz to a new genus, of which no other fragments have yet been found, but which has been named provisionally _Placothorax paradoxus_.] Geologists have still a great deal to learn regarding the contemporaries of the _Holoptychius nobilissimus_. The lower portion of that upper formation to which it more especially belongs--the portion represented in our second pyramid by the conglomerate and sandstone bar--though unfavorable to the preservation of animal remains, represents assuredly no barren period. It has been found to contain bodies apparently organic, that vary in shape like the sponges of our existing seas, which in general appearance they somewhat resemble, but whose class, and even kingdom, are yet to fix.[AR] [Footnote AR: These organisms, if in reality such, are at once very curious and very puzzling. They occur in some localities in great abundance. A piece of Clashbennie flagstone, somewhat more than two feet in length, by fifteen inches in breadth, kindly sent me for examination by the Rev. Mr. Noble, of St. Madoes, bears no fewer than twelve of them on its upper surface, and presents the appearance of a piece of rude sculpture, not very unlike those we sometimes see in country churchyards, on the tombstones of the times of the Revolution. All the twelve vary in appearance. Some of them are of a pear shape--some are irregularly oval--some resemble short cuts of the bole of a tree--some are spread out like ancient manuscripts, partially unrolled--one of the number seems a huge, though not over neatly formed acorn, an apprentice mason's first attempt--the others are of a shape so irregular as to set comparison and description at defiance. They almost all agree, however, when cut transversely, in presenting flat, elliptical arcs as their sectional lines--in having an upper surface comparatively smooth, and an under surface nearly parallel to it, thickly corrugated--and in being all coated with a greasy, shining clay, of a deeper red than the surrounding stone. I was perhaps rather more confident of their organic character after I had examined a few merely detached specimens, than now that I have seen a dozen of them together. It seems at least a circumstance to awaken doubt, that though they occur in various positions on the slab--some extending across it, some lying diagonally, some running lengthwise--the corrugations of their under surfaces should run lengthwise in all--furrowing them in every possible angle, and giving evidence, not apparently to the influences of an organic law, internal to each, but of the operation of some external cause, acting on the whole in one direction.]