HOLOCEI'HAUDS.- 



-nSHES.- 



-Selaciuaxs. 



lo3 



SELACHIANS. 



(Plato IG, the six fisiires ; Plate 17, figs. F and G.) 



Tins tcvin, as onipldyetl b}' Cuvier (Se'laciens), 

 inchidcs tlis Sharks and Rays, and was equivalent with 

 him to his second order of Chondropterygians, com- 

 prising those which have Jixcd t/iUs, by which they are 

 distinguished from his lirst order of cartilaginous fishes, 

 which have movable gills. These have been noticed 

 above under the denominations of Acipenseroids and 

 Ilolocephalids. The group is equivalent to the Plar/i- 

 ostoiai of Miillor and Owen. By Agassiz the Ilolo- 

 cephalids are associated with the Gaieodes (or Sharks) 

 and the Batides (or Rays), the three groups being con- 

 sidered to be orders of a c'ass of vertebrals called by 

 him Selachians. This arrangement is, as has been 

 already intimated, merely provisional, and dependent 

 for final adoption on the issue of researches not yet 

 completed by the illustrious naturalist who suggested it. 

 The Selachians so constituted are nearly identical with 

 the same author's Placoids defined in his treatises 

 on Fossil Ichthyology, and the student ought therefore 

 to be told that placoid scales are bard osseous produc- 

 tions of the skin, sometimes in form of plates of large 

 size, sometimes in shape of small points (forming when 

 the skin is prepared for use in the arts the substance 

 known by the name of " shagreen "), and at other 

 times in form of thorns or prickles. All these difler 

 from ganoid scales in the absence of enamel, as well 

 as in their less regular shapes ; and Kijlliker states 

 that the spines of the Selachians are formed of dentine. 

 The author just named says that the internal skeleton 

 of the Selachians differs in type from that of the Ganoids, 

 and from that of all other fishes except the Dermo- 

 pteres, in being merelj' common cartilage, or cartilage 

 indurated by the deposition of earthy matter, but hav- 

 ing no real bone-cells. In structure, therefore, the 

 dermal skeleton of the Selachians is in advance of the 

 internal one. 



The heart of the Selachians consists, as in other 

 fishes, of a single series of chambers, the first of which 

 named the " venous sinus" is a mere receptacle of the 

 venous blood coming from all parts ; the auricle and 

 ventricle which succeed it are considered to bo the 

 more essential parts of the central propelling organ, 

 and beyond the last named is the stem of the arterial 

 system, named generally the arterial bulb, but which 

 in the Selachians is cylindrical. In these fishes the 

 bag or pericardium in which the heart moves commu-^ 

 nicates with the cavity of the abdomen by a canal, 

 which is single in the Ilolocephalids, but which in the 

 Sharks and Rays bifurcates after leaving the pericar- 

 dium. The venous sinus lies within the pericardium, 

 and is bivalvular at its entrance into the auricle ; and 

 there are from two to six rows of transverse valves 

 within the long muscular arterial stem. 



The gills of the Sharks and Rays are fixed or adherent 

 to the walls of the branchial chambers, each chamber 

 having an interior pharjmgo-branchial slit to admit the 



water of respiration from the mouth, and a peripheral 

 orifice to allow it to escape. In the Sharks these 

 external orifices are situated laterally, and are generally 

 five, but in a few cases six or seven ; in the Rays they 

 are on the ventral aspect, and are always five in num- 

 ber. The gills of the Selachians are, together with the 

 scapulo-coracoid arch, farther back than in the osseous 

 fishes, being behind the skull ; and the heart, which 

 occupies the triangular space between the gills and the 

 chevi'on of the coracoids, has consequently a similar 

 more backward position. The ventrals are abdominal, 

 and there is no swim-bladder. All have the inner mem- 

 brane of the intestine expanded near its termination at 

 the vent into a valve, which exhibits one of two types 

 of form. In one tj-pe the intestinal valve is screw- 

 formed or scalariform, making a spiral slope ; in the 

 other the valve is rolled longitudinally on itself, and 

 attached lengthwise to the wall of the intestine ; both 

 forms serving to augment the surface of the com- 

 paratively short intestine. 



The Sharks and Rays received from Dumeril the 

 name of Plagiostomes, and this appellation being 

 equivalent to the Selaciens of Cuvier, was adopted by 

 Jlilller and Henle in their comprehensive Beschreibung 

 der Plagiostomen, which is our chief guide to a know- 

 ledge of these fishes. The arrangement of these ich- 

 thyologists is as follows : — 



The Squali or Shaeks ( Gafcorfcs of Agassiz) are Plagio 

 stomea with lateral gill-openings [Stigmata) ; free snpplemen 

 tary eyelids ; an incomplete scajiulo-coracoid girdle suspended 

 to the soft parts hehind the gills, but having no articulations 

 either with the skull or spine. These fishes do not possess a 

 cephalopterous cartilage. 



A. First division. — Sharks having one anal fin and two dor- 



sals placed far back, the firpt dorsal being over or 

 behind the ventrals. This division is comprised in the 

 family of tlCTLLnD.'K. 



B. Second division. — Sharks having two dorsals and an anal ; 



the first dorsal being situated over the space interven- 

 ing between the pectorals and ventrals. 



1. First subdivision. — Sharks of B group having a blinker 



(or nictitating membrane), but no spout-holes ; and 

 the last or last two stigmata situated over the base 

 of the pectoral. It includes the families Car- 

 cnARiJEDJE; Spyun^d^; and PrijEnodontid.1;. 



2. Second Subdivision — Sharks ol" B group which have 



spout-holes and blinkers ; also the last or last two 

 stigmata situated over the base of the pectoral. To 

 it belong the families Galeid.e ; Scylliodontid^ ; 

 and MusTELiD^. 



3. Third subdivision. — Sharks ot E group with spout- 



holes but no blinkers. It embraces the families 

 LAMNiEDJi : ODONTASPID^ ; Al.0PECLED.iE ; Cestra- 

 ciosTinjE ; and Khinodoktid^. 



C. Tliird division. — Sharks having but one dorsal and one 



anal fin, forming one family, the NotidanidjE. 



r. Fourth division. — Sharks furnished with spout-holes and 

 two dorsals, but having neither blinkers nor an anal 

 fin. The five stigmata are all situated before the pec- 

 torals; and the intestinal valve is spiral. It embracea 

 the families of Spinacid^; ScYMNiDiE; and K(jnATi- 



NjED.E. 



