A COMPARISON OF THE OSTEOLOGY OF THE JERBOAS AND JUMPING MICE. B}' Marcus W. Lyon Jr., Aid, Diri-^ion of MammoJs. The forms considered in the present paper are those that have usu-ally been classed, especially recently, as a faniih' of rodents under the name Dipodida\ as has l)een done by Mr. Thomas in his paper •'On the Genera of Rodents." While there may be strong grounds for such a classification and for the association of the six genera, S//iiut/tf(s, Zajpus^ Dipux., Alactaga^ Platycercomys.^ and Euchoreutes^ of which the first two are each usually put in a separate subfamily^ yet the limited material at hand is sufiicient to show strong osteolog-ical affinities between Zapn^ and Sininilois which has not usually been recognized and which places them in contrast to the rest of the group. The writer has had for comparison complete skeletons of Zajms and an Egyptian D^'pu-^i in the United States National Museum, two skele-tons of difierent species of Alactaga in the American Museum of Natural Histor}', kindly placed at his disposal by Dr. J. A. Allen, and several odd skulls of Zajms^ Z>//>?Ay, and Alactaga^ as well as the skin and skull of the tj^pe of Sminthus flavus in the National Museum. Za2>iis and D/'j^us represent pronounced types, and for that reason, and ])ecause of the more complete material available, are compared at some length. The vertebral column, with the exception of the cervical region, is essentially the same in each genus; the neck is short and weak; the dorsal vertebrae (twelve) present no peculiarities; the lumbar verte-brae (seven), especially the posterior ones, are built on a heavy plan with largely developed neural and anteriorly directed transverse proc-^This genus has been separated into three subgenera by Mr. Pi-eble, North Ameri-can Fauna No. 15, and recently yir. Gerrit S. Miller, jr.. Preliminary List of New York ]Mammals, Bulletin New York State Museum, YI, 1899, pp. 275, .330-331, has raised the subgenus Napaeozapus to generic rank. It differs from true Zapus only in the absence of the minute upper premolars. Doubtless in time many of the subge-nera in the other genera will be thus raised to generic rank. Proceedings U. S. National Museum, Vol. XXIII— 1228 659
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