54 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 3, 



Ichthyosaurus were particularly described, and contrasted in a table 

 of the measurements of the vertebra? in the two Enaliosaurians. 



In the margin of one of the vertebrae there is an angular notch, 

 about a line in depth, which Prof. Agassiz, after a casual examination, 

 considered to be organic rather than accidental, and to indicate an 

 inferiority of structure, as a similar character is common in Pishes. 

 Prof. Jeffries Wyman, on the other hand, regarded the notch as 

 purely accidental, and the result of a fracture, which has also displaced 

 the articular pits of the superior arch. He is also of opinion that 

 the notch would not be sufficiently important, if it were organic, to 

 affect at all the Enaliosaurian character of the remains. 



Microscopic structure. — A microscopic examination of the osseous 

 structure of these vertebrae revealed well-marked reptilian charac- 

 ters, which were fully described and compared with the characters 

 exhibited by microscopic sections of the vertebras of Ichthyosaurus 

 and Plesiosaurus. 



Comparison with other vertebra 3 . — The author then stated that 

 the vertebras of Eosaurus, in their biconcave centres, exhibit a struc- 

 ture which prevails in the class of Pishes, in the Labyrinthodonts, 

 as well as in a few genera of extinct Saurians, and which is seen in 

 existing reptiles only in the Geckos and the Perennibranchiate divi- 

 sion of Batrachians. 



Amongst Pishes, the Plagiostomi were stated to possess vertebras 

 having more in common with those of Eosaurus than others of the 

 class, but that the latter show a much higher degree of ossification 

 in all their parts than the former; also that the osseous structure 

 differs in the two cases, and that these vertebras do not possess, on 

 their articular faces, any traces of those concentric rings which so 

 generally exist on the vertebrae of fossil and recent Sharks. 



Mr. Marsh also alluded to the persistence of the notochord in the 

 Palaeozoic Pishes, and the incomplete ossification of the vertebras in 

 Mesozoic species ; also to the general difference in the osseous struc- 

 ture of these vertebrae and those of Pishes. He therefore rejects 

 that class, and places the Eosaurus among the Reptiles. 



The differences between the fossil in question and reptilian bicon- 

 cave vertebrae, other than those of the Ichthyopterygia, were then 

 pointed out, the author remarking that in the orders Ganocephala 

 and Labyrinthodontia of Owen, either the notochord was persistent or 

 the neurapophyses were anchylosed to the centrum, neither of which 

 characters is to be observed in Eosaurus. The Crocodilia of the 

 Secondary formations differ from it in having the superior arch 

 united to the centrum by suture ; and in the only genus of Sauro- 

 pterygia whose vertebrae resemble it in their proportions (cervical 

 vertebrae of Pliosaurus), the articular faces were flat, or but very 

 slightly concave. 



The characters of the vertebrae of Eosaurus were contrasted at 

 length with those of Ichthyosaurus, and a very close resemblance 

 was found to exist between them, especially in their flattened 

 and subhexagonal form, in their deep and regular terminal cavi- 

 ties, and in the separate state of the neural arch. The most marked 



