992 o The Perissodactyla. 
been drawn inwards by the ectal movement of the lower jaw, so _ 
that the external wall slopes inwards at the point where it forms — 
in the horse line a V with the angle outwards. In some of the _ 
genera (as Isectolophus) a median, inwardly-directed \ is de 
veloped at this point. The posterior end of the wall is turneda 
little outwards in some forms of this series, but this is due toa d 
return or ental movement of the lower molar, of short duration, 
which has also drawn out externally the external end of the — 
anterior cingulum. The two lines of development are also indi- 
cated in the lower molars. The one has Y's on the lower molars, ‘ 
and the other has cross-crests. The development of Y's is due i 
to the development of intermediate tubercles of the superior 
molars. These cause the crown of the inferior molar to pursue 
a curved path across thé upper, so that when the external cusp — 
of the inferior molar is deflected forwards by the intermediate 
cusp of the superior, the inner cusp of the former must alter- 
nate with the external of the same, in Order to pass between j 
internal cusps of the superior molar.’ 
With these preliminary remarks, I give the following synops 
of the families :? 
I. No A-shaped space with the apex external between cusps 
Inferior molars with cross-crests. 
of superior mola 
A. Premolars different from molars. 
a. Superior canine teeth present 
I, Toes 4-3; Age eee ,Lophiodontide. 
Be TO Pie Ea isd e A Tri 
aa. No superior canine teeth. ste 
3. Mastoid bone not exposed in external wall of skull ;.....++-+++ Canopodide. 
AA. Superior molars and premolars alike, with aoe 
superi 
5. ESAR man excluded from the walls of the skull by the con- on 
_ tact of the occipital and squamosal; no superior, canines +++ Rhinoceride. 
aa. Tea cusps of superior molars subequal, distinct- 
6. Superior molars and premolars alike, and with cross-crests ; ae 
oss “Se eeneeerere . eeereeere apiri 
an " 3 
II. The external cusps of the superior molars ica separi E ; 
T with the ki enina; ; inferior molars with crescen eral asp 
A molars; with only one int 
7- “Toes 4-3: a vertebrarterial canal ;.,..... EE 
+ * These relations will be more fully discussed and illustrated in an 
to appear. 
_& ? Modified from that published in the Proceeds. Amer. Philos. Soc, 
oeoneee? 
article 
1881, 
