526 ANATOMY OF VERTEBRATES. 



deficiency of the wall partitioning the orbital from the temporal 

 cavity. The Lemurs, in this defect, indicate the transition to the 

 lower unguiculate Mammalia, ih.Q Galeojyithecus, ^g. 253, offering 

 the last step by the incompleteness of the orbital frame-ring 

 behind. The outlook of the orbits, in the Aye-aye, obliquely 

 forward, upward, and outward, but least so in the last direction, 

 differs significantly from the direct outward aspect of those ill- 

 defined cavities in most Rodents. 



The basioccipital extends to the fore part of the large tympanic 

 bulla3, to abut against which its margins are slightly produced. 

 The occipital condyles are long and narrow. The plane of the 

 foramen magnum forms with the basioccipital an angle of 125°, 

 its aspect being downward and backward. The paroccipital is a 

 low eminence, and the mastoid in front of it is hardly more pro- 

 minent ; neither process extends freely downward. The super- 

 occipital, ib. 3, is a thin plate moulded on the middle and lateral 

 lobes of the cerebellum, and showing outwardly their respective 

 prominences. The petrosal is impressed by the pit for the cere- 

 bellar appendage. 



There is a small triangular interparietal. The basisphenoid is 

 expanded by a large sinus, and coalesces with the presphenoid. 

 The alisphenoid developes the ectopterygoid ridge, extending 

 from between the squamosal and tympanic to the outer side of the 

 entopterygoid ; both plates are imperforate. The natiform protu- 

 berances form deep depressions in the alisphenoid, on each side 

 the flat square platform of the cranial surface of the basisphenoid, 

 in the middle of which is the subcircular pituitary pit. There are 

 no clinoid processes. The alisphenoids join the parietals, which 

 contribute the greatest share to the formation of the calvarium. 

 The tympanic, coalescing with the petrosal, is, together with that 

 element, expanded into an oval bulla on each side the basi- 

 sphenoid. The parietals, 7, impressed from within to transparent 

 thinness by the longitudinal convolutions of the cerebrum, do not 

 exceed half a line in thickness elsewhere. 



The coronal suture crosses the cranium transversely three lines 

 behind the postorbitals : the frontal suture remains, as in other 

 LemuridcB, and, like the sagittal, it is a harmonia. The fore part 

 of the frontals, ii, project a little between the origin of the nasals, 

 and also between the nasals and maxillaries ; they then join the 

 lacrymals, form the upper half of the inner wall of the orbit, and 

 unite behind with the orbitosphenoid, alisphenoid, and parietal. 

 The rhinencephalic fossa is subcircular and large : the median 

 septum is produced into a ' crista galli.' The frontal sinuses give 



