0. C. Marsh on the Remains of a new Enaliosaurian. 9 



structure is firm and compact, as in the corresponding parts of 

 the Ichthyosaurus. Moreover, the articular faces at the extremi- 

 ties of these centra present no indications of the concentric rings, 

 formed by the partial projection of the osseous laminae, which so 

 generally exist on the vertebrae of fossil and existing Sharks. 



In the comparison of these remains of the Eosaurus with those 

 of fishes, the contrast becomes still more marked as we proceed 

 toward the older formations. The ossification of the vertebral 

 column in the Mesozoic fishes was much less complete, and in 

 some orders almost entirely wanting ; while in all the Palaeozoic 

 species, yet discovered, the notochord was persistent, and con- 

 sequently no bodies of vertebrae have been preserved. Nega- 

 tive evidence, therefore, would strongly indicate that these per- 

 fectly ossified remains should be referred to a higher grade of 

 animals. A microscopic examination, also, of these vertebrae 

 shows that they possess a structure essentially different from 

 that seen in the corresponding bones of fishes; it is, then, evi- 

 dently necessary, considering all the points of divergence, to 

 reject that class, and place the Eosaurus among the Reptilia. 



From the extinct Eeptiles possessing biconcave vertebra, with 

 which we have to compare these remains, we may at once set 

 aside the orders Qanocephala and Lahyrinthodoniia of Prof. Owen ; 

 since in these, according to the present state of knowledge, either 

 the notochord was persistent, as in the Arcliegosaurus ; or, when 

 a higher degree of vertebral ossification was attained, the neura- 

 pophyses were anchylosed to the centrum. Among the Croco 

 dihans, nearly all from the Secondary formations had vertebras 

 of the amphicoelian type ; but, inasmuch as the centrum was 

 terminated by very shallow cavities, and had the superior arch 

 united to it by suture, it will be unnecessary to give further con- 

 sideration to this group. In the Sauropterygia biconcave verte- 

 brae also prevail, but they are all more or less elongated, except 

 in the cervical region of the Pliosaurus, wdiere they have nearly 

 the proportions of the present remains. In this case, however, 

 the articular extremities of the centrum were flat, or very slightly- 

 concave, and consequently this genus may likewise be rejected. 



The remaining orders of extinct Saurians exhibiting the bi- 

 concave structure of vertebrae, with the single exception of the 

 Ichthyopterygia, differ so essentially, in their known remains, 

 from these vertebra of the Eosaurus, either in the length of the 

 centrum, in the depth of its co-adapted cavities, or in its union 

 with the neural arch, that we may evidently terminate the com- 

 parison with the important genus on which that order has been 

 founded. 



It will readily be seen from the previbus description, that a 

 very close resemblance exists between these vertebrae and those 



Am. Jodb. Sci.— Secosd Sbkies, Vol. XXXIV, No. 100.— July, 18^ 



