NEW PLESIOSAURIAN GENUS FROM THE NIOBRARA CRETACEOUS OF

NEBRASKA.

 BY S. W. WILLISTON AND ROY L. MOODIE

(Abstract)

     The new genus is a member of the widely distributed family, the Elasmosauridae, and is based on a complete series of cervical vertebræ with limbs and girdle elements. The distinguishing characters of the new genus are the short, uniform centra of the cervical vertebræ from Cloud County, Kansas. The characters of the limbs show the animal to have been a young but nearly mature individual. There are no evidences for the formation of the articular surfaces of the ends of the propodials for articulation with the mesopodial elements. The cervical centra have on the ventral surface throughout a pair of vascular foramina, and the surfaces of all the skeletal elements show evidence of the presence of a richly vascular periosteum. There are 51 cervical vertebræ preserved with a total length of nearly two meters. The individual vertebræ increase in length and breadth according to the following measurements:

     3d cervical vertebræ     20 mm. long        30 mm. wide
     6th     "             "            23   "       "          32    "     "
   12th     "             "            30   "       "          38    "     "
   20th     "             "            34   "      "          44    "     "
   30th     "            "            41   "       "          52    "     "
   40th     "            "            45   "       "          67    "     "
   45th    "             "            45   "       "          67    "     "
   51st     "            "            46   "        "         72    "     "

The new generic and specific terms Ogmodirus martini are proposed for the new animal.
     The limbs are short and heavy, with the ends of the propodials not modified for the reception of the mesopodial elements. The anterior and posterior propodials both exhibit traces of the peculiar vascular canals and cavities so often described for the plesiosaurs. In the opinion of the junior author this condition is a persistence of an embryonic condition confined to the plesiosaurs and indicating a distinct evolution in that group. The developing long bones of all vertebrates show precisely the same condition as is exhibited by the adult propodials. The fact that they seem to be more exaggerated in some embryonic plesiosaurian propodials would go to show that the character has been deeply impressed on the organization of some of the group. Other members of the Plesiosauria show this character very little, or not at all. Until the plesiosaurs are better known, no definite solution of the problem can be attained; but it is suggested that the more generalized and plastic forms of the group will be found to exhibit this character, while the development of this character has been obliterated in other forms.