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sometimes cylindric, and somtimes [sic] narrowed. The surface from a short distance above the alveolar margins to the tip, is marked with acute thread-like ridges, which are sometimes interrupted, and some-times furnished with short branchlets. They are more or less undulate, and do not unite, but simply cease as the tip of the tooth is approached. The latter is smooth without lateral cutting edges. The width of the mandible at the commencement of the rami is 3 in. .05 lin.; of the muzzle of the seventh tooth 3 in. 7.5 lin.; at the third tooth 2 in. 4.2 lin.

General Remarks.—The tail is a powerful swimming organ, more or less compressed in life, hence the specific name, which means flat-tailed.

The danger of injury to which such an excessively elongate neck has been exposed, would render the recovery of a perfect specimen like the present an unusual chance. The neural spines of the dorsal region are so elevated and closely placed as to allow of little or no vertical motion of the column downwards, while those of the cervical and caudal region being narrower, the elevation of the head is quite possible, and an upward flexure easy.

The habit of this species, like that of its nearest known allies, was raptorial, as evinced. by the numerous canine-like teeth, and the fish remains taken from beneath its vertebræ.

The general form of this reptile, whether it was furnished with large posterior limbs or not, was that of a serpent with a relatively shorter, more robust and more posteriorly placed body than is characteristic of true serpents, and with two pairs of limbs or paddles. It progressed by the strokes of its paddles, assisted by its powerful and oar-like tail. The body was steadied by the elevated keel of the median dorsal line, formed by the broad, high neural spines. The snake-like neck was raised high in the air, or depressed at the will of the animal, now arched swan-like preparatory to a plunge after a fish, now stretched in repose on the water or deflexed in exploring the depths below.

Differences from other Sauropterygia.—The only genus with which it is necessary to compare this present one is Cimoliasaurus. The following may be noted as generic distinctions: The series of cervicals rapidly diminishes in Cimoliasaurus in absolute size and in relative length of the vertebræ, which are not compressed. In the present genus they maintain a similar length for a considerable distance, diminish in length very gradually and are much compressed. The diapophyses of the dorsal vertebræ, as they descend, in Cimoliasaurus, continue well developed until they attain the inferior planes of the centrum, and have there a downward direction. In Elasmosaurus they cease while yet on the middle of the centrum, and are replaced by pits throughout the remainder of the length.

The neural canal is everywhere markedly larger in Cimoliasaurus.

As the characters of lesser significance may be added, that in Cimoliasaurus magnus the dorsals with elevated diapophyses are considerably larger in the eentra than those in which they are situated lower down. In E. platyurus these vertebræ are of relatively equal length.

The cervical pleurapophyses in C. magnus are anteriorly considerably stouter and less flattened; the same applies to more anterior vertebrae, where they are flatter in both.

In comparing this species with the Cimoliasaurus grandis, Leidy, from Arkansas, we observe, first, the generic character of the strong inferior diapophyses in the latter. That species marks itself also as a preeminently short-necked form, as these anterior dorsals are even shorter than in C. magnus, being nearly twice as wide as long. The depth of the articular faces is also relatively greater than in the E. platyurus.

History.—The determination of the extremities of this species was rendered difficult from the fact that Leidy in his descriptions of Cimoliasaurus, reverses the relations of the vertebræ, viewing the cervicals as caudals and lumbars, and describing the caudals as belonging to another genus. Not suspecting this error, I arranged the skeleton of Elasmosaurus with the same relation of extremities, and the more willingly as the distal cervicals present an extraordinary attenuation, even for this type, and also as the discoverer assured me that the fragments of cranium were found at the extremity which is properly the caudal. Viewed in this light many details of the structure were the re-