NO. ICiX. 



OSTEOLOGY OF THE JUMPING MICE— LYON. 



661 



The tibia is essentially the sauK^ in each, thouo-h T)i]niH has a larger 

 crest in front. 



The li])iila is slender, long and distinct above, as usual in the Myo- 

 uiorphs, l)ut fuses tirmly with the til)ia below, a little above its middle 

 in />//>//6"and about as far below the middle in Zap ax. 



The tarsus is composed of the same elements in each genus, but is 

 diti'erentlv arranged in each, as — 



Zapus has the anterior nonarticular 

 part of the astragakis rather elongated, 

 thus pushing the navicular forward, so 

 that the outer side is in contact with the 

 cuboid. 



In Zapus the internal cuneiform is not 

 much elongated and ends in an articular 

 surface for the tirst metatarsal. 



DipHsha»the corres]ionding part of the 

 astragalus shortened, so that the navicular 

 appears somewhat shut off from the, 

 cuboid. 



In Dipus the internal cuneiform is dis- 

 proportionately long and lies close against 

 the second metatarsal, ending in a thinned 

 extremity. 



The metatarsal bones show striking dilierences. 



In Dipus there is but one long i-ounded 

 bone, "cannon" Ijone, trilid at its distal 

 extremity, where it presents three articu- 

 lating surfaces for the three digits. It is 

 to l;>e regarded as a compound bone com- 

 posed of three fused metatarsals. 



The two lateral digits are subequal and 

 but little shorter than the middle one. 

 Each is composed of three phalanges. 

 There is just a trace of metatarsal five, on 

 the outer posterior part of tlie "cannon" 

 hone. 



In Zapus they are five in number, 

 elongated and separate, the lateral ones 

 being subequal, but decidedly shorter 

 than the three central ones. Each meta- 

 tarsal bears a digit. 



All the digits have three phalanges 

 except the innermost, which bears but 

 two as usual. 



The three middle digits have the rela- 

 tive proportions seen in Dipus, a slightly 

 longer median one and two subequal lat- 

 eral ones. The innermost or first digit, 

 hallux, reachesonly as far as the metatarso- 

 phalangeal articulation of the middle toes, 

 and the first phalanx of the outer or fifth 

 toe reaches the same point. 



The skulls, as a whole, show very little resemblance to one another, 

 points of communit}' being found in the maxillo-zygomatic region 

 onl}'. The skulls of the D'lpodUliv {Dl2n(s, Alactagd, Platyeercoinys^ 

 Euchoi'i^uits^ Sininthus, and Zapu)<) are characterized chiefly by the 

 great development of the antorbital foramen, large and rounded and 

 witli a more or less separate canal for the transmission of the .superior 

 maxillary division of the trigeminal nerve. The malar consists of a 

 more or less horizontal portion articulating with the squamosal pos- 

 teriorly and with the maxilla anteriorly, and a large more or less 

 vertical portion, the anterior edge of which is in contact with the 

 maxilla, the posterior edge free and forming the anterior boundai'v of 

 the orl)it, while the superior end of the vertical part is in contact with 

 the lachrymal. But thest^ common characters present several impor- 

 tant differences in the two o-enera. 



