ELASMOliHAMIl." 



107 



projt'i'ts (lowiiwMrils ill an aiifjle, known as tlie basilar 

 an'^l«"- ''"' '" ^'i^' 111"!'** I'econt forms of Sharks and 

 in tlic l!a\s tlu'se articulations iiave been looseiuHl, and 

 are r(']ilai('d by ligaments, the postorbital process and 

 the basilar plate having meanwhile suffered reduction''. 

 The Oiiiniieras have undergone an opposite development, 

 the palatoijuadratc cartilage (fig. 294, qpfj) licing con- 

 fluent with the skull — whence the nanic of Ifuloi-e- 

 nhuVt — and furiuinu a firm floor fo the orbit. \\'itliiii 

 the skull, above and just behind the basilar angle, lies 

 the Tiirl.isli saddle (with the mfimdihuluin, gJcnidida 

 pitiiitiniii. and ccDiaUs caroticus), and here the septum 

 between the orbits is |>ierced by a transverse canal, in 

 and nround which the niiiscali recti of the eye have 

 tlieir insertions, and wliich is consequently homologous 

 with the posterioi- orliito-muscular canal present in most 

 of the Teleosts. In the orbital region the roof of the 

 skull (tlu' forehead) is sometimes (in the; Kays) iraper- 

 fectlv closed, lieiiig completed by a membrane (frontal 

 fontanelle, fig. 300, fonf). The anterior limit of the 

 orbits is formed bv the preorbital process (tigs. "2i)<S — 

 300, proh), which is pierced, as well as the more or 

 less arched and expanded roofs of the orbits (with their 

 supraorbital foramina, fig. 298, fspo), by ramifications 

 of the ophthiiliiiic lu-anch of the trigeminal nerve. The 

 canal (preorbital canal, figs. 294 and 29(S, cp) that 

 traverses the upper part of the preorbital process tor 

 this purpose either opens on the top of the skull (fig. 

 299, cp^) and soon descends again into the cartilage 

 of the ethiiKiid region (ethmoidal canal, tig. 294, re), 

 or proceeds uninterruptedly, without appearing on the 

 surface, into the said cartilage, or leads to a deep in- 

 cision on each side of the skull (fig. 299, ie). the limit 

 between the orbital and etlimoid regions. In the lower 

 part of the preorl)ital jirocess runs another canal, the 

 so-called orbitonasal canal (tig. 294, on), to receive an- 

 other ramification of the said ophthalmic nerve. Behind 

 the ojjtic foramen there often projects within each orbit 

 a cartilaginous rod to support the ej'eball, this rod being 

 the so-called eve-stalk. The tip of each preorlutal pro- 

 cess .sends out in the Xotidanida (the most primitive in 

 type of the surviving Sharks) a backward process, 

 which ill the lia\s (fig. 300, poh) becomes a free car- 

 tilage jutting out laterally and jointed l>y ;i well-deve- 



loped articulation with the ]ireorl)ital j)rocess and the 

 nasal ca|)sule. \\\ I'akki;!:' this cartilage was inter- 

 lireted as a \estige of a preoral (situated before the 

 moiitli) \isceral arch; but if would seem to have most 

 in common with the preorbital bone of the Teleosts, 

 es])ecially as this appears in the CnUtidce. In the 

 lia\s it meets the prorsal basal cartilage (propte- 

 rvgiuni) of the pectoral fins, and is most highly de- 

 veloiied in the Electric b'avs (Torpediiii-s) and Kagle- 

 Hays {MjiHubatidd). 



The ethmoid region composes the anterior limit 

 of the cranial cavity, being the jiroduct of the strong 

 expansion and development of cartilage attained by the 

 side-angles of the embryonic trabecuhe (fig. 298, ctr). 

 Medialh" this mass coalesces into a septum, the inter- 

 nasal cartilage, between the large nasal capsules (figs. 

 294 and 298 — oOO, N), each of which is pierced with 

 a large foramen for the olfactory nerve. On the upper 

 side of the skull there appears at this point, in most 

 of the Elasmobranchs, the large prefrontal fontanelle 

 (figs. 299 and 300, fonpf), filled with a loose, mucoid 

 connective tissue, the backward continuation of which 

 passes, in the interior of the skull, into the hard mem- 

 brane of the brain {dura mater). The lateral parts of 

 the ethmoid region are fornuid by the large nasal cap- 

 sules (iV), which ai'c of an inverted bowl-shape, arched 

 above, open below, and the ))osterior walls of which 

 coalesce on each side with either preorbital process. 

 The structure of these capsules originates, it is true, 

 from the side-ends of the embryonic trabecuke; but 

 also includes up to three so-called labial cartilages 

 (figs. 294 and 298, /«, and In.,), originally free, which 

 form the margin and marginal valves of the nostrils. 



The rostral region, which attains its greatest de- 

 velopment in the Hays and Saw-fishes — being pro- 

 duced in the latter into the so-called saw with its large 

 lateral .serrations and its two pairs of longitudinal ca- 

 nals, the inner pair continuations of the above-men- 

 tioned preorbital canals — is least developed in the 

 most primordial Sharks, where it forms a compressed 

 and narrow, oi- dejiressed and l)road prolongation of 

 the cranial floor. In most Sharks, however, the snout 

 is furnished on each side with a siijiporting cartilage 

 (figs. 298 and 299. Jr). originally a so-called labial 



" Cf. tlie Sturgeons, see above p. 1045, tig. 284, U. 



'' An exception to tliis is tl.e Hammerhead (.Zygceiia), in wliich tlie postorbital jirocess as well enters into the laleraUy elongated orbits. 



' StcMll of Sharks, Shates, Trans. Zool. Soc. Lond., vol. X, p. 224 (ao). 



