8 O. C. Marsh on the Remains of a new Enaliosaurian. 



and ramify, as in the bones of fishes, nor anastomose with the 

 corresponding tubes from the neighboring cells, although in one 

 of the longitudinal sections there are a few indications of such 

 a connection. Some of the other sections examined show a 

 larger number of canaliculi than those in Plate II. ; but generally 

 there are only a few of these tubes attached to each lacuna, and 

 in some cases they appear to be entirely wanting. As the cana- 

 liculi vary much in number in different saurians, and also a 



the age of the animal, their paucity in this case is not remarka- 

 ble. It is possible, however, that the method employed in pre- 

 paring the sections was not well adapted to rendering these 



) tubes visible. In a part of the t 

 in Plate 11. figure 5, a structure is seen which is quite different 

 from the surrounding osseous substance. This may be due to 

 the presence of a small cavity in the bone before the introduc- 

 tion of the mineral matter, or to an imperfect ossification at that 

 point : more probably the latter, as these vertebrae, like those of 

 the Plesiosaurus, show in their interior structure a degree of 

 ossification somewhat inferior to that at the articular terminal 

 surfaces. 



The vertebrae of the Fosaurus, in their biconcave centers,' 

 exhibit a structure which prevails in the class of Fishes ; 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 division of Batrachians. These vertebrse, 

 however, present such marked characters in their very short 

 antero-posterior diameter, in their deep and regular terminal con- 

 cavities, and in the separate condition of their neurapophyses, 

 that, in determining the position to which their anatomical fea- 

 tures entitle them to be assigned, we may safely limit our com- 

 parison to the Fishes, and to those genera of extinct Saurians 

 which possessed similar characters. 



In comparing these remains with the former class, there is at 

 once apparent a much closer resemblance in the above respects 

 to the vertebrse of the Plagiosiomi, than to those of any other 

 order. The remains of these fishes have been found in all fosail- 

 iferous strata above the Lower Silurian, but no vertebrse except 

 in the more recent rocks. In the Cretaceous and Tertiary forma- 

 tions one family of this order, the S(iualidcje, have left numerous 

 and well preserved vertebrse ; and the writer has carefully com- 

 pared the remains of the Fosaurus with a large number of these 

 and other similar fossils, but could find little resemblance except 

 in form. The vertebrae of the Fosaurus show a much higher 

 degree of ossification than those of fishes ; and this extends as 

 well to the non-articular surfaces as to the terminal facets of the 

 centers. In the former, there are none of the cavities which are 

 found in the vertebral surfaces of this class ; but the osseous 



