THE TILLODONT SKELETON SKULL

In a discussion of the tillodont skull one is limited largely to the end products,

Trogosus and

Tillodon of the middle Eocene, as the earlier form, Esthonyx,

though known by more numerous

remains,

is actually far less well represented so far as quality or completeness of the material is concerned.

The Wasatchian

materials are for the

most

^part

more

poorly preserved, often crushed, distorted,

and

have a mineral encrustation that can be satisfactorily

removed

with only the

most

painstaking preparation.

Of

the fragmentary skull portions

known,

other than maxillae,

two

fair, but exasperatingly incomplete specimens are

known from

the

Wind

River stage.

A

^few,

much-

encrusted skull fragments,

among

the

many

dentitions, are included in lower

Wasatchian

or

Gray

Bull materials,

and

the Clark

Fork

has furnished only maxillae

and

lower jaws.

The

tillodont skullpossesses

an

archaic appearance, sharedby

many

of the

Eocene and

Paleocene

mammals,

but ^is unusual in its modifi- cation of the rostral portion

accompanying

the

tremendous

^enlarge-

ment

of thesecondincisors. Initsgross

form

therostrumis elongate,

somewhat

tapering, slightly to

markedly

concaveabove,

and

anteriorly resembling that of a

hedgehog on

a

much

larger scale.

The

ventral surfaceisbroad, particularly inthe posterior portion, across thelarge, relatively brachydont

molar

^teeth,

and

is moderately concave between the toothrows.

The

frontalareaisespeciallybroad,

and

dorsally

more

or less inflated, giving the skull, particularly that of T. grangeri (pi.

NO. 10

TILLODONTIA — ^GAZIN

53 6), a "high

brow"

appearance.

The

orbits are

low and

^{widely sepa-}

rated,

and

the

zygomae

well expanded.

The

postorbital processes are

weak

but the temporalcrests well defined,

though

less prominently in the T.hillsii skull (pi.9),

forming

the anterodorsal extent of the deep temporal fossae.

The

sagittal crest, developed to varying

amounts

anteriorly, has a

pronounced

saddle about

midway

of its length, over the

narrow

braincase,

and

risestoconsiderable

prominence

posteriorly as it joins the

heavy

lambdoidal crests. Ventrally, the pterygoids project prominently

and

the basicranialarea posteriortotheseisvery short

and

broad.

The

occipital area, as observed in T. grangeri, has

an

outline that is almost semicircular,

though

slightly acute dorsally,

and viewed from

the sides is nearlyvertical in the lowerpart, but^the dorsal half curves

backward toward

the inion (pi. 6).

Ingreaterdetail, therostrumof thetillodont skull exhibitsa

number

of features

which

should be brought out, in addition to the peculiar dental specialization characterizing the order.

Beginning

withthe dor- sal surface,the nasals arevery elongate,extending well back

between

the frontals.

Forward,

the sides of the nasals are nearly parallel or

somewhat

converging, but posteriorly these bones

widen

markedly, with the greatest width

between

the maxillofrontal sutures. Posteri- orly, the nasals

may

terminate in a

broad V-shaped wedge between

the frontals, as in T. fodiens (pi. 13), or

form

a broad, nearly rec- tangular reentrant as in T. grangeri (pi. 5) or T. lullsii (pi. 9).

The

skull referred to T. hyracoides (pi. i)

shows

an intermediate form, with a noticeable pit atthe apex, probably

due

to

an

injury.

The

premaxillaoccupies

an

unusuallylargeproportion of thelateral

and

dorsolateral surface of the snout. In its surface expression, this

bone

extendsposteriorlytoa pointabout even with apositioninterme- diate

between

the infraorbital

foramen and

theorbit.

The

premaxillo- maxillary suture extends

from

itsposterior limit obliquely across the faceto a midposition

between

the third incisor

and

the canine, result- ingⁱⁿa surfacedistribution for the premaxilla

which

isapproximated, but not quite reached, inthe living hedgehog, Eriimceus.

Between P

and C

the suture is deflected to the canine alveolus (not as

shown

in

Marsh's

illustration, 1876, ^pi. 9)

and

then extends abruptly forward, passing close to P, to the anterior palatine foramen.

The

premaxilla

is prominentlyflexed over the unerupted portion of

P which

extends throughthis

bone and

terminatesposteriorlyinthemaxillaataposition slightlyabove

and inward from

the anterioropeningof the infraorbital foramen. Anteriorly, the premaxillae join in a pair of

backward-

directedwings,partially orcompletelyseparating the anterior palatine foramina,

and

producing a

wide

palatalnotch opening

forward between

54 SMITHSONIAN MISCELLANEOUS COLLECTIONS

VOL. 121 the large incisors.

The

anterior portion of the nasal cavity, as noted intheT. hyracoidesskull,

shows

anearly lemniscate outlineinaverti- cal section, in

which

prominent ridges on the mesial walls of the premaxillae,

presumably

for attachment of the maxilloturbinals, par- tiallyconstrictthe nasalpassageintoupper

and

lowerchambers.

These may

correspond tothemiddle

and

inferiormeatusesof the nose.

The

superior meatuses are definedby thearcuate

form

of the inferior sur- face of the nasals as seen

from

in front. Inthe T.hillsiiskullthe nasal section observed is

more

posterior in position,

showing

a less pro-

nounced

horizontal stricture,

and

in

which

the outline of the section is

somewhat more

bell-shaped, with the nasal fossae partially separated

by

the sutural ridge of the maxillae,

and

possibly

by

a part of the vomer.

In a ventral aspect of the rostrum it has been observed that the maxillae

widen

very greatly

from

the position of P^

backward

^;

how-

ever,withthe increase in size of the cheekteethposteriorly, theefifec- tive width of the palate between the lingual margins of the teeth in- creases but little

from

the incisors back.

The

anterior

margin

of the maxillae

and

the anterior palatine foramina are far

forward on

the palatal surface, in a position

median

to the third incisors, almost as far

forward

as in Erinaceus.'^

Weak

posterior palatine foramina appear inthe customaryposition atornearthesuturebetweenthemaxillae

and

palatines,but a

nonsym-

metrical scattering of such foramina occur

forward

of this position,

and

a pair of much-better-developed foramina, with well-defined for-

ward

continuinggroovesaretobe

found median

tothe thirdpremolars in threeof theskulls,

and

opposite the second premolarsin T.fodiens

(pl. 15).

The

nasal

wing

of the maxilla

makes

but a short contact with the nasal

bone and

is excluded

from

participation in the anterior

margin

of the orbit

by

the lachrymal

and

jugalbones, except inthe T. hyra- coides skull

where

the edge of the maxilla

forms

a part of the rim.

The

anterioropeningof the infraorbital canalisabovea pointbetween P^

and

P*. Posteriorly its openinginto the orbital fossais very large

and bounded

above

by

the lachrymal

and

below

by

the orbital plate of the maxilla.

The

ascending plate of the palatine does not appear totake partintheforamen.

The

orbital plateof the maxillaisa broad shelflike structure of considerable extent,

and

a conspicuous feature of thetillodont skull

when

viewed

from

above

and

behind.

'^In making comparisons with the European hedgehog, it should be borne in

mind thatthehighlyspecialized incisors are probably nothomologous inthetwo forms. This tooth is

P

ⁱⁿ the hedgehog.