ORDER IX. 
PLAGIOSTOMI. 
The sharks and rays, which are brought together under this heading, have, as common char¬ 
acters, an internal skeleton, cartilaginous or partly ossified, the vertebral column exhibiting 
greater or lesser indications- of transverse separations; the cranium constituting a continuous mass, 
in which the individual parts are not recognizable; the jaws being likewise cartilaginous and 
attached to the cranium by cartilages also. The teeth are situated on the roof of the mouth 
and on the lower jaw. The anterior part of the head is prolonged forwards; under the snout, 
at a greater or lesser distance from its extremity, may he observed a broad transverse mouth, 
in advance of which the two nasal fossae are situated. The gills are fixed, and the branchial 
apertures five or more in number ; the spiracles being not always present. The scapular or 
thoracic arch is not attached to the head. The ventrals and pectorals are always extant, 
although soft and fleshy like the other fins; the ventrals placed posteriorly to the pectorals. 
The swimming or air bladder is wanting. The intestine is provided with a spiral valve; the 
external investment consisting of shagreen or small indurated plates variously modified. 
Syn — Plagiostomes, Dum. Zool. analyt. 1806 ; &, Ichthyol. analyt. 1856, 113 & 120. 
Selaciens, Cuv. Regn. Anim. II, 1817, 121; 2d ed. II, 1829; &, ed. illustr. Poiss. 356.— Bonap. Sagg. Distr. method 
Anim. Vert. 1831, 121.— BeKay, New Y. Faun. IV., 1842, 348. 
Plagiostomi , Mull, in Wiegm. Archiv fur Naturg. 1845, I, 137.— Storer, Synops. 1846, 250.— Bd. Iconogr. Encyl. 
II, 1850, 205 and 240. 
Selachii, Richards. Faun. Bor. Amer. Ill, 1836, 287. 
The order of Plagiostomi divides into two subordinate groups—the sharks on one side, and 
the rays on the other. The distinguishing features of either of these sub-orders will be 
recalled under their respective headings. 
It may not be amiss to state that, under the name of Selachians (Selachii), were formerly 
included the representatives of the orders Plagiostomi and Holocephala of the present day. 
SUB-ORDER I. 
SQUALL 
The sharks which constitute this sub-order are slender, elongated, fusiform or sub-fusiform; 
a thoracic arch incomplete; pectoral fins distinct from the head, and situated on the sides of the 
chest, as in most osseous fishes. The eyes are provided with free or movable eyelids; the 
branchial fissures being lateral, situated either entirely in advance of the anterior margin of the 
pectorals, or a portion of them may extend over the base of these latter mentioned fins. Verte¬ 
bral column exhibiting transverse divisions throughout. 
Syn. — Squalidae, Bonap. Sagg. Dist. method. Anim. Vert. 1831, 121; Syst. Vertebr. 1837, 45; &, Selach. Tabul. anal. 
1838, 4.— DeKay, New Y. Faun. IV, 1842, 348.— Mull, in Wiegm. Archiv. fur Naturg. I, 1845, 137.— 
Storer, Synops. 1846, 251. 
Squali, Mull. Vergl. Anat. Myxin. I, 1836, 75.— Mull. & Henle, Syst. Beschr. Plagiost. 1841, 1. 
Pleurotremes, Dum. Ichthyol. analyt. 1856, 120. 
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