NATURAL SCIENCES OF PHILADELPHIA. 313 



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 squamosal; and certainly the 

 enlarged extremity of this ridge, behind the meatus, has every ap- 

 pearence 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 suturation is with 

 the mastoid, excepting the basioccipital. The occipital lies in 

 three planes, nearly at right angles with each other. The basi- 

 occipital is horizontal, as usual upon the floor of the skull; the 

 exoccipitals, with probably part of the supraoccipital, are perpen- 

 dicular, behind ; the rest of the supraoccipital is horizontal again, 

 on top of the skull. The basioccipital is wedge-shaped, and offers 

 nothing very peculiar, excepting its entire disconnection from the 

 petrosals between which it lies ; its sphenoidal articulation is just 

 behind the joined apices of the petrosals. Exoccipitals appear as 

 a pair of flaring flange-like processes, just outside the condyles, 

 appressed against the otic capsules. The foramen is very large, 

 subcircular, and mostly in the perpendicular plane of the bone. 

 After inclosing this orifice, the bone rapidly contracts as it rises 

 to the top of the skull ; this part is marked with a sharp perpen- 

 dicular median ridge, and the edges of the bone being affected 

 in coming into apposition with the swelling mastoids, there results 

 a pair of deep narrow fossce upon the face of the bone. The por- 

 tion of the supraoccipital which mounts the top of the skull to 

 there lie horizontal, almost immediately forks to embrace a small 

 shield-shaped interparietal bone between its prongs. The ends 

 of these prongs touch posterior corners of the parietals 



The sphenoid bone is of rather small extent, owing to the situa- 

 tion of the squamosals in the orbit. It is widely fissured from 

 the petrosals. The alisphenoid is very short; its termination 

 may be seen in the jagged suture with the squamosal ; but short as 

 it is, it only misses taking part in the mandibular articulation, since 

 it extends to the margin of the glenoid. The orbito-sphenoid is 

 comparatively smaller still, the place it occupies in Geomyidse 

 for instance being here largely occupied by the squamosal. 

 The spheno-palatal suture may be traced in young specimens 

 with a zigzag but still in general transverse course, from the side 

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