ORDER IX. 



PLAGIOSTOMI. 



The sharks and rays, whicli 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 or^ 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 be 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. 



SYS — Plagiostomes, Ddm. Zool. analyt. 1806 ; &, Ichthyol. analyt. 1856, 113 & 120. 



Selaciens, Gov. Regn. Anim. II, 1817, 121 ; 2d ed. II, 1829; &, ed. illustr. Poiss. 356.— Bonap. Sagg. Distr. method 



Anim. Vert. 1831, 121.— BeKat, New Y. Faun. IV., 1842, 348. 

 Plagiostomi, Mull, in Wiegm. Archiv fur Naturg. 1845, I, 137. — Storer, Synops. 1846, 250. — Bd. leonogr. 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 arcli 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.—S^ua/idae, Bonap. Sagg. Dist. method. Anim. Vert. 1831, 121; Syst. Vertebr. 1837, 45; &, Selach. Tabul. anal. 



1838, 4.— DeKat, New Y. Faun. IV, 1842, 348.— Mcll. in Wiegm. Archiv. fur Naturg. I, 1845, 137.— 



St9rer, Synops. 1846, 251. 

 Squall, Mull. Vergl. Anat. Myxin. I, 1836, 75.— Mull. &, Henle, Syst. Beschr. Plagiost. 1841, 1. 

 Pleurotremec, DuM. Ichtliyol. analyt. 1856, 120. 



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