18841)! 521 [Cope, 
possess the widely different: position ascribed to this genus by Leidy. Few 
ifany of the characters given by Mr. Bettany as those of the genus, can 
be regarded as other than characters common to several of its ‘species. 
Perhaps the most important of these is the angle formed by the basifacial 
with the basicranial axis, by which the face is presented as much forwards 
.as upwards. The species present considerable variety in form. The ge- 
nus embraces the largest species of the family, such as I. macrostegus, M. 
superbus, etc. The characters of the species are as follows : 
I. Foramen infraorbitale above middle of fourth superior premolar ; pose 
terior part of zygoma expanded ; palate moderately produced posteriorly. 
Squamosal part of zygoma less expanded anteriorly and with 
rounded border; head elongated; premaxillary bone not 
produced ; otic bulla larger, compressed, extending anterior 
to postglenoid process ; size large .......eceecsecseceeeedl, superdus, 
Head shortened occipitally, so that a line drawn through post- 
glenoid and paroccipital processes makes 90° with the 
middle line; malar bone openly grooved. below orbit; 
angle of mandible obliquely truncate. ....ecssereccenceees M. leidyt. 
Lacey part of zygoma most expanded in front, and elevated 
behind, so that the cranium is as wide as from the paroccipi- 
tal process to the canine tooth ; its posterior angle rising to a 
level with the sagittal crest; its inferior edge spread out- 
wards ; its superior edge truncated ; occiput not shortened ; 
malar flat below orbit ; postglenoid process marking front 
OLFOUUIM Ee inevebiviiene ld aways Hie MAVGe s10,8i8r6 whbctih ik puma melas UA chelydra.. 
II. Foramen infraorbitale above the first true molar. Palate greatly 
produced posteriorly. 
Squamosal part.of zygoma much expanded, and with truncate 
edge; malar bone robust, prominent; skull, width equal 
length from condyles to first premolar ; maxillary produced 
anteriorly ; frontal plane, transverse diamond-shaped ; 
bulla small, conical, posterior to anterior edge of postglenoid 
PTOCOSS.. cece eee cee reece eee enecerseerecceseseees «Dh, macrostegus. 
Squamosal part of zygoma little expanded upwards or lateral- 
ly, edge rounded; malar bone flat; bulla large, extend- 
ing in front of postglenoid process; front longitudinally 
diamond-shaped, decurved at orbit...... teeoeereeciev un Dh, montanus 
III. Foramen infraorbitale above anterior border of second 
true molar. 
Zygoma originating above second molar; large; incisors 
AMA CUS MBI Y Aye Gish tis a sip a sloidial bin avsn Wie koru man ei snngtlel URTICUDD 
Zygoma originating above third true molar; larger; incisors 
Ne xeOx GACG TOL VDE iinnn's vise din Wp adua tease iecaraioraentiye Wh yyarelere's M. proprius. 
Of the above seven species, four are represented in my collection, some 
of them by a large amount of material. The latter are from the John 
