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Boyer diss 2009 1046..

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Figure 2.35. UMMZ TS13 Lagostomus maximus (A-C): A, ventral view of skull. B, inset<br />

of right tympanic cavity. B’, enlargement of inset B. B”, enlargement of inset in B’. C,<br />

inset of anterior end of bulla. C’ enlargement of inset C. There is distinct color difference<br />

visible near where petrosal/ectotympanic boundary must be in B’, B” and C’. Note that<br />

in B” more medial (farther right) tca foramen represents opening of tympanic canaliculus<br />

into tympanic (bullar) cavity. tca foramen to left is beginning of shallow canal that<br />

represents continued lateral course of branches of tympanic nerve. AMNH 41522<br />

Lagostomus maximus (D-E’): D, ventral view of left petrosal. Bulla has been cut away in<br />

this specimen. E, inset of medial aspect of promontorium. E’, enlargement of inset E.<br />

Unlike Marmota, Lagostomus only has grooves on its promontorium relating back to<br />

tympanic canaliculus in wall of jugular foramen (internal carotid plexus apparently does<br />

not enter tympanic cavity from medial position in Lagostomus). In E’ tca foramen<br />

farthest to left represents initial opening of tympanic canaliculus into bullar cavity. Next<br />

foramen to right represents beginning of canal for continued course of nerve, while that<br />

farthest to right represents end of this short, shallowly floored canal. Grooves and canals<br />

leading from here across promontorium appear similar to morphology in this region in<br />

plesiadapids and in Marmota. Numbers and abbreviations: Bas – basisphenoid ; Boc –<br />

basioccipital ; bs – bullar suture ; ccA – aperture for cochlear canaliculus ; Ect –<br />

ectotympanic ; Ptr – petrosal ; tc – tubal canal ; tca – tympanic canaliculus.<br />

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