The description, otherwise, shows no discrepancies of importance. The chief difference given by the author is the size, and this character we think our specimens show to be of little specific value. “It is a question of some importance how far difference in size among the Mosasauroids may be a test of difference in species. Among the numerous remains of these animals which have been discovered I have never yet observed any which presented any evidence relative to age. * * * In this view of the case, some of the many described species of Mosasauroids may have been founded on different sizes of the same.”[9] [9] Leidy, Rep. U. S. Geol. Surv. Hayden, vol. i, p. 284. The length of the cervical vertebrae in the specimen above described is thirty-seven or thirty-eight millimeters. The cervical vertebrae in two specimens referred to _C. pumilus_ have lengths respectively of twenty-two and thirty millimeters. In the type specimen of _C. velox_ they must have had a length of at least forty-two millimeters. It thus appears that, between the smallest specimen, which, in life, could have hardly exceeded eight feet in length, our specimens, indistinguishable anatomically, represent forms of ten and twelve feet, while the type itself was about fifteen feet in length. Of the material originally referred to _C. pumilus_, there are in the collection five or more specimens, which, altogether, furnish nearly every part of the skeleton. They present no tangible differences from the skeleton of _C. velox_ described above. There can be, hence, little or no doubt but that the name _C. pumilus_ is a synonym. It is hardly possible to say with certainty that _C. affinis_ Leidy is or is not the same as _C. velox_, but, so far as the description goes, we can find few differences. The type is of about the same size as the type of _C. velox_, and the figures agree well with the bones of the skeleton described. Although the description was not published till 1873, the author makes no mention of the species of Marsh’s. Leidy describes the back teeth as having the enamel strongly striated, with the surface presenting evidences of subdivision into narrow planes. In this respect, only, it disagrees with the specimen.