AND AVES OF NORTH AMERICA. 



39 



penetrating the centrum, which, though not wanting in such mammals as Basilosaurus and 

 its allies, arc neither so numerous nor situate so near the neural arch as here. (See Leidy's 

 fig. LI.) 



The ribs, as remarked by Dr. Leidy, are remarkably dense, lie observes that "from 

 the solidity of structure and cylindroid form of the ribs, I suspect Ischyrotheriwn to be 

 more nearly allied to the Manatee than to any other animal." This consideration dees 

 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 homoge- 

 neity, 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* this concentric structure is eccentric in relation to 

 the circumference of the rib. 



The genus Mesosaurus Gervais, according to plate XLII of his Zoologie el, Paheontol- 

 ogie Gencrale (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 

 1 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, 1 have thought that the name applied by Leidy, which is appro- 

 priate only to a mammal, should be (hanged. I therefore call it Ischyrosaurus, maintain- 

 ing the first etymology so far as practicable. 



I refer it to the kSauropterygia, as the parts resemble Plesiosaurus more nearly than 

 those of any other American genus. The density of the osseous structure and the cylin- 

 dric form of the ribs, will distinguish it from Plesiosaurus ; from Polycotylus the form of 

 the vertebrae separates it at once. 



I suppose; that this type may have been of estuary habits, and took its food in proxim- 

 ity to hind. The density of the bones is not known in, nor is it appropriate to, animals 

 of the open ocean. The presence of Iladrosaurus (Tliespesius) occidentals in the same 

 beds, is further evidence of the proximity of land. 



ISCHYROSAURUS ANTIQUUS, Leidy. 

 T&ohyrotheriurn, antiquum, Leidy. Proo. Ac. Nat,. Sci., Phila., VIII, p. S'.). Trans. Amer. Philos. 800. 18(10, 150 



'Pali. X, dgS. 8-17. 



The dorsal vertebra 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 



* Seii Owen on this irenus. 



