OF THE SKULL IN SHAEKS AJS^D SKATES. 223 



rest is trabecular. On the upper surface it is easy to see how much building-material 

 has been used to form the superorbital arcs ; whatever their morphological nature may 

 be, they are parts that do not chondrify separately, but are very distinct in early 

 embiyos, as I have shown in my paper on the Salmon. Behind, the crested form of the 

 auditory masses is seen to be largely due to the form of the great loops into which the 

 internal sac is developed : the primary opening, or vestibular " aqueduct " (aq.v.) is only 

 covered by skin. The interorbital space above is still retained as a large oblong fonta- 

 nelle ; then there is a short and narrow bridge of cartilage, and then, in front of that, 

 the lanceolate opening into the great prsecranial or internasal vacuity. On each side of 

 this gaping space the trabeculae, as they melt into the rostrum, grow over as an inter- 

 nasal eave. These ascending and upper growths of the trabeculse represent, by a sort of 

 morphological hypertrophy, the trabecular crests of a bird or mammal that form the 

 lower portion of the mesethmoid. The nasal sacs, very large and wide apart, are 

 modified from their original dome-shape by the condyle on their outside for the rib- 

 shaped antorbital (a.o), and by the addition of a new stratum of cartilage yielded by the 

 great superorbital arc. The same arc still shows its form behind, where it has thickened 

 the periotic mass, and formed the sphenotic process (sp.o). Between the superorbitals 

 the skull is flat above ; and below, the basis cranii is only very gently convex. 



The occipital condyles (oc.c) project but little ; and a tract of notochordal jelly is 

 interposed between the median part and the first vertebra. The nerve-outlets are most 

 of them best seen in the side view (PI. XLI. fig. 4. 2, 5, 7). The peculiar mode of 

 suspension of the double dentigerous arch is best seen in the side view (PI. XLI. fig. 4, 

 q.jjq, mn, mt.pcj, hn) ; the hyomandibular running forwards to the base of the metaptery- 

 goid, or spiracular cartilage, to be attached by ligament to both the quadrate and the 

 angle of the mandible. The loose spiracular cartilage bears but little of the weight of 

 the jaw ; of which it is the proper apex. 



In the lateral view the arches are compressed as much as possible for display ; in the 

 upper and lower views they are in a state of rest, and therefore greatly depressed. 



The pterygo-quadrate bar is attached in front by ligament behind the nasal sac ; and 

 the ligamentous fibres attached to the round inferior end of the oblique subtrihedral 

 hyomandibular spread fan-like, to insert themselves on the top of the quadrate and the 

 angle of the mandible. The latter is hinged in a somewhat complex manner, a ball on 

 the quadrate slipping into an articular cup, which has three nodules on its rim, whilst 

 the whole mass of the angular region is thin-edged. 



The articular part of the upper bar — the qua,drate — is separated by a cleanly made 

 neck from the proper pterygoid region ; but no inspection of the parts in the adult 

 would have led to the discovery of the fact that this upper dentigerous bar, in its 

 toothed part, is a mere process or fork of the mandibular or first postoral bar. The 

 posterior part of the hyoid is seen to be jointed by its epihyal part [e.liy) to the end of 

 the condyle of the hyomandibular ; the ligament is almost the length of the so-called 



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