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]
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.
Plioplatecarpus Dollo is described by its author as having a sacrum of two conjoined vertebrae,[10] by reason of which it is placed in a separate family from the rest of the Pythonomorpha. It may be presumptuous to express a doubt of the genuineness of the sacrum, and yet, save from the fact that the author found two specimens quite alike, one might doubt it strongly. It is not very rare that two, or even three vertebrae are found united from injury in these animals, and such would readily account for the consolidation as figured and described by Dollo, except for the coincidence of the second specimen. A stronger reason for doubt is the statement that the consolidated vertebrae belong to the posterior “lumbar” region, and that the last vertebrae had small tubercles indicative of chevrons. In the reptiles which we have examined, the chevrons do not begin immediately behind the pelvis, but are separated by a longer or shorter region in which the vertebrae bear elongated diapophyses alone. If the conjoined vertebrae figured by Dollo are in reality sacral, it would appear that the animal is an exception to Clidastes and such lizards as we have examined. Furthermore, the pelvis must have been of a different structure from that in the Kansas genera of the Pythonomorpha, for, in these, it is evident that the ilium had an oblique position, and could have been attached to but a single diapophysis.
CLIDASTES WESTII, N. SP.
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.