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Show 208 ON THE STRUC'fUHE OF THE SKULL. The class Pisces presents us w·i t1 1 a co n1pleto serjos of these crani.a froin t h e 1e as t ossi'f i e d forms ' which possess o. nly one or two pa'.n ·s of carti.1 a ge bo nes I· n the walls of the cranium, to the Fig. 84. N-Mn . . 4 L . 'd ·. A the parieto-frontal bone ; B , tl1c supra-orbital. ; . C,, the nasal; Fig. 8 .- ept osu en .. '{l'o{d. E the vornerine teeth ; E . 0 ., the ex-occipital ; Mn, the D, th~ pal~rpt~~ l1 oid . Br the branchiostegal ray ; Op, the opercular plate ; x, tmhaen pdaibrales p;I 1 enoy1~ dt ; y, yth e bone ~hich gives atta1c hment to the scapular arch; Or, the orbit; Au, the anJitory chamber ; N, the nasa sac. completely ossified skulls of the Oyprinoids and SDuroids. And again, just as among the preceding gro~ps w~ found that th~ Ohi1nmroids differed widely from the rest In havin? th~ sub-ocular process, or arch of the sku~l, to whi~h tl:e manc:Ible Is a~tached, formed of one piece of cartilage, winch IS continuo:us ';Ith, .and immoveable upon, the skull; so, in this series, L ep2dos2ren IS at once distinguished from all the rest by a similar character. The skull of the Mudfish (Fig. 84) is composed of a framework of cartilage, which sends down a broad ~ri angular process, on each side to articulate with the mandible, and expands, posteriorly and laterally, into chambers for the auditory organs. Between these, the roof and the floor of the skull are both constituted by cartilage; but anterior to the1n, as far as the. extremity of the parasphenoid (x), this tissue becomes very thln or disappears (Fig. 85). In front of the anterior end of tho parasphenoid it makes its appearance again on both the roo~ and the floor ' of the cranial cavity, beyond w hw' 11 I' t I·S co nt1nucd as a thin lan1ella to the end of the snout. A fibrous septum withc. a free concave, posterior margi•n , a·I VI· a es t·h 1' s re gion of the cranium into two lateral chambers, one for each olfactory lobe. THE SKULT,S OF FISHES AND AMPHIBIA. 209 Behind the auditory chambers the cartilage is almost excluded fro1n the walls of the skull by two lateral ossifications of its substance-the ex-occipitals (E.G.). As in tho A1nphibia, there is no ossified supra- or basi-occipital. The rest of the lateral parietes of the skull would be devoid of bony walls were it not that the parasphenoid (x) and the great bone (A), which roofs in the whole length of the skull, and answers to the frontals and parietals, send upwards and downwards, respectively, lateral processes, which unite together, and so replace the alisphenoid (Fig. 85). The ethmo-vomerine cartilage (Eth. Vo.) bears; superiorly, the nasal hones (0), and inferiorly it carries teeth (E). A long flat bone, pointed posteriorly (B, B), is attached to the hinder edgB of the nasals, and roofs over the orbit and temporal fossa. Fig. 85. Fig. 85.-T.ougitudinal and vertical section of the Skull of Lept"dusiren. The cartilage is dot~ed; the me~branous and bony constituents are shaded with Jin es. A, B, C, D, as m the precedmg figure; x , x , the parasphenoid; 1, 2, the fi rst and second ver~ tebral arches; Ch, the notochord ; Au, the situation of the aurlitory organ. The notochord, which forms the chief axis of the spinal column of this fish, is continued into the base of the skull, and ends in a point about the level of the exit of the trigen1inal nerves (V). There is neither basi-occipital nor basi-sphenoid, and the presphenoid is represented only by the cartilaginous floor at (1!.8'). The pterygo-palatine apparatus is represented, ?n eac~ s1de, by t~e great dentigerous curved plate. (D), which Is apphed to the Inner surface of the cartilaginous sub-ocular process, abuts against the parasphenoid by its inner edge, and clesce~ds to the inner side of the articular condyle for the m~n.dible (a). The hyoidean arch (Hy) is attached to the illHtdle of the posterior and inferior edge of the sub-ocular p |
