Order RODENTIA.

2554 specimens examined

It was not possible to undertake a complete survey of this order; the small size of many specimens rendered examination of the teeth too difficult. However, rodents seem to be remarkably free from positional irregularities of the teeth; only 11 (0.4%) were affected.

Family Sciuridae

In the genus Marmota (marmots), the P3 are sometimes situated a little lingually to the other cheek teeth; in some specimens, this is accentuated and the mesial surface of P4 slants mesially. This was seen in 14 of the 97 specimens examined, but in only one was the misplacement marked (Fig. 14.1).

In Petaurista (giant flying squirrels), the P3 are small and situated a little lingual to P4; the position of M3 varies. Rotation of P4 with the buccal surfaces facing mesially (Fig. 14.2) was seen in three out of 148 mandibles.

622 specimens of Sciurus (squirrels) were examined; there were no irregularities.

Family Ctenomyidae

395 skulls of Ctenomys (tuco-tuco) were examined; in one (C.fratersylvanus, BMNH 1921.1.2.10), both M3 were more lingually situated than normal.

Family Octodontidae

In Octodontomys gliroides (mountain degu), the buccal surfaces of the upper cheek teeth are progressively turned more and more distally from the distal to the mesial ends of the arches, with P4 being nearly transverse to the line of the arch (Fig. 14.3).