52S 



MONOORAPnS OF NORTH AMERICAN RODENTIA. 



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process of the squamosal is of peculiar character; instead of a slender curved 

 epuv reaching around to grasp the malar, there is a short abrupt heel appresscd 

 against the tympanic, and to the roughened face of this heel the clubbed end 

 of the necdic-likc nminr is affixed. The relation of the part« is such that the 

 zygoma appears to articulate l)ehind with the tympanic — it actually has an 

 abutment ogninst that bone, though no real articulation with it 



From tile lower back edge of the squamosa!, a curious thread of bone 

 starts off* and occupies the deep groove already mentioned as se|)arating the 

 tympanic from the mastoid. No break from the squamosal can be seen in 

 this thread, which curls around the orifice of the meatus, still in the groove 

 mentioned, and ends by a slightly enlarged extremity below and behind tlio 

 meatus, exactly in the position of an ordinary "mastoid process". I am 

 uncertain of the meaning of this. The end of this ligule or girdle of bone 

 thus encircling the tympanic is in the site of the iiostero-lateral angle of the 

 skull in Ofomtfida, in which such angle is formed by a corner of the squamo- 

 sal ; and the inference is self-suggestive that this delicate bony strap may 

 rcully be squamosal — an edge of the squamosal persisting in situ after the rest 

 of that bone has been crowded down into the orbit by the encroachment of 

 the mastoid. Such a view, however, will bear further scrutiny. Even if a 

 slender spur of actual squamosal does run out into the tympano-mastoid 

 groove, it does not follow that the whole of the fold in this groove is squamo- 

 sal ; and certainly the enlarged extremity of this ridge, behind the meatus, 

 has every appearance of an ordinary mastoid process. 



Next after the squamosal, the occipital bone suffers most from the 

 enlargement of the otic elements ; it is singularly restricted in extent, and 

 presents itself in unique shape, compressed between the swollen mastoids. 

 All the lateral occipital saturation is with the mastoid, excepting the basi- 

 occipitul. The occipital lies in three planes, nearly at right angles With each 

 other. The basioccipital is horizontal, as usual, upon the floor of the skull; 

 the exoccipitals, wi(h probably part of the supraoccipital, are per|)endicular 

 behind^ the rest of the bupraoccipital is horizontal again, on top of the skull. 

 The basioccipital is wedge-shaped, and offers nothing very peculiar, except- 

 ing its entire disconnection from the {letrosals, between which it lies; its 

 sphenoidal articulation is just behind the joined opices of the petrosals. ■ 

 £xt)ccipit«l8 appear as a pair of flaring flange-like processes, just outside the 

 condyles, apprcdsed against the otic capsules. The foramen is very large, 



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