penetrating the centrum, which, though not wanting in such mammals as Basilosaurus and its allies, are neither so numerous nor situate so near the neural arch as here. (See Leidy's fig. 11.)
The ribs, as remarked by Dr. Leidy, are remarkably dense. He observes that "from the solidity of structure and cylindroid form of the ribs, I suspect Ischyrotherium to be more nearly allied to the Manatee than to any other animal." This consideration does not affect the affinities here accepted as true. The structure is remarkable, and differs from that of Manatus and Squalodon much as Reptiles do from Mammals, in its homogeneity, or when interruption of the same occurs, in its appearance as irregularly disposed cells, and in the lack of a concentric structure of any kind. In the Mammalian genera in question, as well as in Basilosaurus[1] this concentric structure is eccentric in relation to the circumference of the rib.
The genus Mesosaurus Gervais, according to plate XLII of his Zoologie et Palæontologie Generale (the letter press has not yet reached me), presents ribs of similar form to those of Ischyrosaurus, but whether of similar structure I cannot ascertain.
Leidy concludes his description of this genus with the following remarks : "Although I have supposed the remains * * to indicate * * an animal allied to the Manatee, * * I have suspected that they have belonged to an aquatic reptile unlike any known." * * Entertaining the opinions that I do respecting the relations of the genus, I have thought that the name applied by Leidy, which is appropriate only to a mammal, should be changed. I therefore call it Ischyrosaurus, maintaining the first etymology so far as practicable.
I refer it to the Sauropterygia, as the parts resemble Plesiosaurus more nearly than those of any other American genus. The density of the osseous structure and the cylindric form of the ribs, will distinguish it from Plesiosaurus ; from Polycotylus the form of the vertebræ separates it at once.
I suppose that this type may have been of estuary habits, and took its food in proximity to land. The density of the bones is not known in, nor is it appropriate to, animals of the open ocean. The presence of Hadrosaurus (Thespesius) occidentalis in the same beds, is further evidence of the proximity of land.
ISCHYROSAURUS ANTIQUUS, Leidy.
Ischyrotherium antiquum, Leidy. Proc. Ac. Nat. Sci., Phila., VIII, p. 89. Trans. Amer. Philos. Soc. 1860. 130 Tab. X, figs. 8-17.
The dorsal vertebræ of this species present plane articular extremities. The centrum is not constricted medially. but presents a shallow concavity round its median portions. The sutural articulation of the neural arch is shallow. sub-ovate, and extends throughout the length of the centrum. The diapophyses are compressed cylindric. The
