A specimen of much interest in the University collection differs so markedly from the other forms represented by specimens, as also from the descriptions of the known species, that we are constrained to regard it as new. It was collected by Mr. C. H. Sternberg from the uppermost of the Niobrara beds, in the vicinity of the old town of Sheridan. The character of the associated invertebrate fossils seems to indicate a different geological horizon, either the Fox Hills group, or transition beds to that group. The specimen consists of a complete lower jaw, quadrate, portions of the skull, the larger part of the vertebral column, and the incomplete hind and fore paddles. The vertebrae preserved are in two series, the one, numbering thirty-three, continuous with the skull; the other, sixty-three in number, all chevron caudals. The terminal caudals preserved indicate that there were several more in life, perhaps five or ten; the first of the series was evidently among the first of those which bore chevrons. Altogether the tail may have had seventy-five chevron caudals. The lengths of the two series are respectively seventy-one and seventy-two inches. Assuming that there was the same number of precaudal vertebrae as in _C. velox_, the entire vertebral column would have measured in life fifteen feet and four inches. The lower jaw shows the skull to have been very nearly twenty-four inches in length, making, for the animal when alive, a length of seventeen and one-half feet. This is one of the largest species, and it is interesting to observe that the real size here, as usually elsewhere among fossil vertebrates, is less than supposed. It is doubtful whether there is a _Clidastes_ known that exceeded twenty feet in length. While the skeleton was only about one half longer than the specimen of _C. velox_ described in the foregoing pages, or of about the same length as a very complete specimen of _C. tortor_ in the museum, the proportions of the animal were very much stouter. The figures given in plate VI of the twenty-fifth, or eighteenth dorsal, vertebra will show the relations between length and breadth: it is upon these remarkably stout proportions, and the shape of the articular faces, as indicated by the figures and by the measurements appended, that the species is chiefly based. The articular surfaces of the basal caudal vertebrae are remarkably triangular in shape, with the angles rounded, and the sides of nearly equal length. This triangular shape is persistent for the first twenty of the series as they are preserved. The paddles, as shown in plates IV and V, show much stouter proportions than in either _C. velox_ or _C. tortor_. The species comes nearest to _C. stenops_ Cope, but it seems hardly the same. It is, also, evidently allied to _C. dispar_ Marsh. From these and other described species, the following, extracted from the original descriptions, will serve to show the differences, in comparison with the specimen of _C. Westii_.