3. On Elasmosaurus platyurus Cope; by Prof. E. D. Cope. (Communicated by the author).- I observe, in the last number of this Journal, that Prof. J. Leidy criticizes my determination of the structural characters and generic relationships of the above reptile, stating that I have reversed the direction of the vertebral column, describing the cervical as the caudal series and vice versa and that it is the same as his previously described genus Discosaurus. On these points I would make the following observations.

     First, as to the direction of the vertebral column, I have little doubt that Prof. Leidy is correct in his determination, especially since I have already pointed out, that, assuming the direction I gave it to be true, the vertebral articulations, and the scapular and pelvic arches, appeared to be the reverse of those of Plesiosaurus. Prof. Leidy does not, however, allude to the principal cause of this error, which was the similar reversal of the vertebral column in his descriptions of his genus Cimoliasaurus, first published in 1851, and re-published with 2 plates in 1864. Having my mind pre-occupied with this determination and not suspecting the error, I arranged Elasmosaurus in accordance with it. The great size of the clavicle, and lack of special characters of the scapular arch, as mesosternal, etc., and consequent close resemblance to many reptilian fishes, rendered the error more easy, while the coincident discovery of several reptilian forms with zygosphene articulation, attracted my attention to that character.

     It might be added that the description and restoration are correct in the "Synopsis Extinct Batr. Reptilia, etc., North America," the error having appeared in a few extra copies only. Also that the accounts already given will require scarcely any modification, the caudal region like the cervical being very long, and less depressed than in Cimoliasaurus, etc.

     Second, from the identification of Elasmosaurus with Discosaurus, I entirely dissent. Dr. Leidy, having assumed the cervicals of Cimoliasaurus to be lumbars, and stating it as "probable that part of the series described as lumbars may be regarded as representing sacrals and caudals,"* referred the true caudals of the same genus, to another supposed genus, under the name of Discosaurus. Anterior caudals of Cimoliasaurus magnus he regarded as cervicals of the new genus. But, entertaining a suspicion that the two genera might be one, he says that in this case, they "represent cervicals, dorsals and lumbars of Discosaurus," (i. e. Cimoliasaurus, the name earliest given). Having shown the identity of the two forms in accordance with the structure of Elasmosaurus, I failed to reverse the arrangement

* Cretaceous Reptiles, N. America. 29.

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adopted by Prof. Leidy. Had I done so, Cimoliasaurus and Discosaurus would have become synonyms of Plesiosaurus, since no characters are known by which to distinguish them. As it is, I preserve the first, on the supposition that its scapular arch, will be found to present the peculiarities belonging to Elasmosaurus.

     There are, however, several species included in Leidy's last description of Discosaurus vetustus, as he suggests, but believing that "the material was not sufficient to justify a separation," he allowed them to remain together. A portion of this material from New Jersey belongs undoubtedly to Cimoliasaurus magnus, and the other specimens (two vertebræ), which present a few peculiarities, are recorded in my Synopsis Extinct Batrachia, Reptilia, etc., as Cimoliasaurus vetustus. I presume that it is on these two vertebræ that Leidy bases his reference of Elasmosaurus to Discosaurus. If the evidence furnished by these was "insufficient to justify their separation " from C. magnus, it is certainly insufficient to justify their reference to another genus. The proximal caudals of Elasmosaurus and Cimoliasaurus are identical, but the median and distal caudals of the two are quite distinct. In Elasmosaurus they present a deep median groove beneath and a rib-like elevation on each side. No such vertebræ have been described as referable to Cimoliasaurus, and there is no evidence to prove that the slightly angulate caudals among those referred to C. vetustus by Prof. Leidy did not belong to the medial caudal region of a Cimoliasaurus.* In a little notice furnished to LeConte's report on the Geology of the Union Pacific R. R., southern division, written almost as soon as I received the fossil, I temporarily referred the caudals to Discosaurus, not being generally willing to establish a new genus on caudal vertebræ or other distal portions.

      In conclusion, it may be summarily stated that: 1, Discosaurus was erroneously constituted; 2, that characters separating it from Plesiosaurus were not adduced; 3, that it was not distinguished from Cimoliasaurus; 4, that Discosaurus vetustus embraces at least two species, one of which is Cimoliasaurus magnus; and, 5, the other cannot be proven to be an Elasmosaurus, but scarcely differs from corresponding parts of Cimoliasaurus magnus.