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ME. W. K. PAEKER ON THE STEUCTUEE AND 



are the sphenoidal structures complex. As in other Rodents, there is a large presphe- 

 noid as well as a basisphenoid ; the alisphenoids are ossified from two centres on each 

 side; long "lingular" pedicles are formed by the apices of the trabeculae, and to these 

 are articulated a pair of long, outstretched falcate bones, the evident counterparts of the 

 " basitemporals " of the Bird. In this animal also the " external pterygoid processes " 

 are hasipterygoids, and the small pterygoid bone is attached to their aj^ex. Even in the 

 Ruminants these spurs are hasal in their origin. (For the development of the human 

 sphenoid, see Huxley, 'Elem. Comp. Anat.' p. 144.) 



Sixth Stage. — Emhnjo Pigs, 6 inches long, measured from snout to ischium. 



The head in this stage equals in size that of a Squirrel, but its bones are much more 

 dense. The roof-bones (Plate XXXV. figs. 1-3) are now applied to each other edge 

 to edge by sutures, and in certain places overlapping as squama?. The " anterior fonta- 

 nelle " {fo.) is still open, but is much lessened ; the parietal and occipital bones now form 

 a good " lambdoidal suture." The nasals, frontals, parietals, squamosals, lacrymals, pre- 

 maxillaries, maxillaries, and malars (n.,f., ])., sq.. I., px,, mx., j.) are all so far formed as to 

 require but little change of size to fit them for their adult relationships. The palatal 

 region (Plate XXXV. fig. 2) shows a great development of the secondary floor or 

 " hard palate," the palatines themselves being now tied together at the mid line below. 

 The ossification of the cartilaginous skull has advanced greatly ; the superoccipital is a 

 large, strong shell of bone, and it is at present separated by a larger tract of cartilage 

 from the exoccipitals than in the last stage (s.o., e.o.). The latter have now only one 

 centre, for the epiphysis formed primarily in the substance of the condyle has coalesced 

 with the outer deposit ; this is now creeping far down into the substance of the long 

 twisted paroccipital process {p.oc). The basioccipital [h.o.) now reaches from the 

 foramen magnum to the spheno-occipital synchondrosis; and in front of it (fig. 2) 

 the basisphenoid is now a thick mass of bone (see also fig. 3, h.o., b.s.). The presphe- 

 noidal region is hidden below by the vomer (fig. 2, v.), but in the section (fig. ^,p.s.) it 

 is seen to be largely unossified. The alisphenoids (figs. 3 & 4, al.s.) are solidly anchy- 

 losed to the median piece ; they are larger relatively to the orbito-sphenoids, but 

 are still inferior in size and in place; they largely owe their size, laterally, to the 

 external pterygoid processes (e.pg.), for their cranial region is small. The whole orbital 

 wing is much more contracted, relatively (figs. 1, 3, 4, o.s.) ; it has become detached 

 from the ethmoid, and is some distance from the auditory mass. The two centres are 

 completely anchylosed at the mid line, and quite enring the optic passages (2); below 

 (fig. 3), they are forming the presphenoid. 



The remainder of the facial axis and nasal septum is one sheet of solid cartilage ; 

 and so also is the complex nasal labyrinth, now much more complex in its turbinal 

 growths and cribriform plate {u.fh., cr.p.). 



From the intimate impaction of the auditory mass into the sides of the cranium, its 

 osseous centres have caused much confusion ; this has, however, been greatest in the 



