OF THE SKULL IN SHARKS AND SKATES. 199 



edge of the outspread apex can be seen just above the pituitary body {py). At present 

 the azygous basi trabecular rod is beneath the hemispheres (6'i, a) ; but afterwards it 

 forms the axis of the beak or cutioatei: On the whole, tlie sectional view of the 

 second stage is much like that of the first ; but a dissected head shows that changes of 

 the utmost importance have taken place. The segmentation of the proper branchial 

 arches will be shown better in the next stage ; they all, save the last, break up into 

 four pieces on each side — a "pharyngo-," "epi-," "cerato-," and "hypo-branchial" element. 

 But the mandibular and hyoid arches (PI. XXXVI. fig. 1, mn, hy) undergo only one 

 transverse segmentation. This is but a step above what occurs in the Lamprey (Huxley, 

 "Elem." p. 193, fig. 76, g), where the mandible does not become subdivided at all, and 

 the hyoid arch is only severed across to form two pieces. Our next subject, the Skate, 

 is far in advance of this ; and on this point the ordinary Shark is not in advance of the 

 Cliimcera. It has recently been shown that in Osseous Fishes the hyoid arch is much 

 more complex than its successors, the proper branchials (" Salmon's Skull," pis. 2, 3, 4, 

 5, 6 & 8), being composed of the same number of cartilages ; but three of these are 

 partly segmented, each into two, by having an additional bony centre. In the Sturgeon 

 (' Monthly Microsc. Journ.' May 1873, pi. 20. fig. 1) there are Jive cartilages on each 

 side. Here, then, the " pharyngohyal " and the " epihyal " are in one piece, and the 

 " ceratohyal " and " hypohyal " are in one. Here also the 7node of segmentation is 

 different and altogether simpler : in the bony fish it is from top to bottom, the second 

 postoral bar being longitudinally segmented (" Salmon's Skull," pi. 2. fig. 3) ; but in the 

 Shark it is transverse segmentation, exactly like that of the bar in front, namely the 

 mandibular. Whilst the pedate process of the mandible runs and fastens itself to the 

 trabecula by its apex, the hyoid applies its pedate process to the whole side of the 

 auditory capsule on which it is hinged ; but the distal part of the upper piece is strongly 

 tied to the arch in front of it. This simplicity of the oral arches occurs in the " Dipnoi," 

 and also is the pattern on which the " Urodela " are constructed, although in both 

 these cases fibrous bones complicate the structure ; but in the Teleostei, as a rule, and 

 in Ganoids, both " Chondrostei " and " Holostei," the oral arches have to be traced up 

 from those of the Rays, which are complex ; these are to be described presently. 



Whilst the apex of the hyoid arch as well as the pedate process keeps attached 

 above, the apex of the mandible becomes a mere fibrous band in front of the "spiracle," 

 or first postoral cleft. This band is attached to the skull behind and below the fifth 

 nerve (fig. 1, cl. 1). Here is the beginning of that peculiar modification of the Fish's 

 skull by which the mouth becomes so mobile, the mandibular and hyoid arches hanging 

 from the head by a single suspensorium — the " hyomandibular," — which may be either 

 the whole of the upper part of the hyoid arch, as in the present instance, or the larger 

 half of it, obtained by longitudinal segmentation, as in the Salmon ("Salmon's Skull," 

 pi. 2. fig. 3, h.m). 



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