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ME. W. K. PAEKEE ON THE STEUCTUEE AJ^D 



coils. The bilobate mass directly above the trabecular horn {al.tb., i.tb., c.tr.) has its 

 anterior lobe developed into the curious " alinasal turbinals " immediately within the 

 snout, whilst its hinder lobe becomes the long " inferior turbinal." The swelling below 

 the down-turned roof is the rudiment of the " nasal turbinal," scarcely developed in the 

 adult of this type ; and the mass which lies beneath the rudiment of the olfactory crus 

 (1) becomes the upper and middle turbinal (one mass in the Pig) and the true olfactory 

 region. Vertical sections* show, most instructively, what could in nowise have been 

 guessed at — namely, that the nasal labyrinth has its skeletal parts formed by the approxi- 

 mation and coalescence of two imperfect cylinders open freely below, and by these 

 receiving at their junction below the ascending crest formed by the conjugated trabeculse 

 (Plate XXIX. figs. 1-3, a/.s., al.e., tr.). When these trabecular bands continue flat 

 (as in the embryo, Plate XXIX. figs. 1 & 2, tr.), then we have, as in the Frog and the 

 Crow, a cartilaginous floor to the nasal passages (see memoir on " Frog's Skull," Plate vii. 

 fig. 6, and Plate x. fig. 3, s.n.l. ; and also Proc. Roy. Micr. Soc, Oct. 2, 1872, p. 224, 

 pi. 38. fig. 1, s.n.). In most Mammals, and in Birds not belonging to the Passerine 

 group, the trabeculee narrow in to form the rounded thickened base of the whole " ethmo- 

 presphenoidal bar;" this process is seen to be beginning in the section through the 

 posterior part of the nasal region (Plate XXIX. fig. 3, tr.). The section through the 

 inner nares (Plate XXIX. fig. 3, i.n.) also shows the back wall of each nasal passage 

 [j).n.w.). These large rounded spaces are seen to have the rudiments of the last of the 

 middle turbinal coils already continuous with the end M'all. These posterior M'alls 

 correspond to the end of each " sphenoidal sinus ;" this is therefore the presphenoidal 

 region, and behind the mesoethmoid ; and the pyriform openings above were made 

 through the front of the cranium and through the fore end of the cerebral hemi- 

 spheres, where they bud-off" the olfactory crus (see also Plate XXVIII. fig. 6, CI", ol.). 

 This same section has cut througli the first cleft (first preoral or lacrymal passage) on 

 its way to the nasal passage. The anterior extremity of the, as yet, soft palato-pterygoid 

 rod (ppg-) is here cut through, where it passes below the inner nostril (i.n.). The 

 space between the rudimentary olfactory crus and the budding upper turbinal (Plate 

 XXVIII. fig. G, oL, s.n.) is composed, at present, of an almost structureless gelatinous 

 stroma; it is slow to form those cartilaginous bands afterwards, which, creeping between 

 the olfactory filaments, form the cribriform plate — a secondary morphological structure 

 almost entirely peculiar to the Mammalian skull. 



The eyeball only affects the skull from Avitliout, by modifying the facial and cranial 

 structures to form its safe recess or orbit ; but the earball is constructed after the 

 fashion of the old cottage oven, being built into the side v/alls of the skull, bulging out 

 on the outside, and having its nerve-mouth pi-ojecting also within. 



In my last paper, on the Salmon's skull, I was able to show the infolding of the 



* These vertical sections of the nasal region were made by the same slicing as the horizontal sections of the 

 head further back : this depended upon the hooked shape of the head at this stage ; the razor passed at right 

 angles to the nasal roof, but parallel to the notochord. 



