29. On the Missouri near the Platte, occur masses of rock, which seem to be almost exclusively composed of a remarkable petrifaction, belonging to the family of concamerated shells. This shell is elongated, fusiform, and when broken transversely, it exhibits the appearance of numerous cells disposed spirally as in the _nummulite_, but its longitudinal section displays only deep grooves. The shell was therefore composed of tubes or syphons, placed parallel to each other, and revolving laterally, as in the genus _melonis_ of Lamarck, with which its characters undoubtedly correspond. But as in the transverse fracture, its spiral system of tubes cannot be traced to the centre in any of the numerous specimens we have examined, it would seem to have a solid axis, and consequently belongs to that division of the genus that Montfort regards as distinct, under the name of _miliolites_, which seems to be similar to the _fasciolites_ of Parkinson, and altogether different from the miliolites of Lamarck. Our specimens are conspicuously striated on the exterior, which distinction, together with their elongated fusiform shape, sufficiently distinguish them as a species from the _sabulosus_ which Montfort describes as the type of his genus. No aperture is discoverable in this shell, but the termination of the exterior volution very much resembles an aperture as long as the shell. The length is three-tenths of an inch; and its greatest breadth one-twelfth. We call it _miliolites secalicus_, SAY.--Mr. T. Nuttall informs me, that he observed it in great quantities high up the Missouri. In the same mass were some segments of the encrinus, and a terebratula with five or six obtuse longitudinal waves. 30. Another petrifaction, abundant in some fragments of compact carbonate of lime, also found on the shores of the Missouri, possesses all the generic characters which we have attributed to the preceding species, excepting that in the transverse fracture the cells distinctly revolve from the centre itself, and of course the shell was destitute of the solid nucleus as in melonis, _Lamarck_. It has about four volutions. We have named this species, which is, notwithstanding the difference of the central portion of the same genus with the preceding _miliolites centralis, Say_. As in the preceding, it is entirely filled solidly with carbonate of lime, and this substance being of a greater purity in the filled-up cavities of the fossil than in the mass, its interior divisions are very obvious. The latter species we observed about one hundred miles up the Konzas river, where it forms the chief body of the rocks in extensive ranges. It seems to be a carbonate of lime containing iron.--JAMES.