Holockphauds.- 
-FISHKS. tiELACHlANS. 
SELACHIANS. 
(Plate 16, the six figures ; Plate 17, figs. F and G.) 
This term, as employed by Cuvier [Selaciens), 
includes the Sharks and Rays, and was equivalent with 
him to his second order of Chondropterygians, com- 
prising those which have fixed gills, by which they are 
distinguished from his first order of cartilaginous fishes, 
which have movable gills. These have been noticed 
above under the denominations of Acipenseroids and 
Holocephalids. The group is equivalent to the Plagi- 
ostomi of Muller and Owen. By Agassiz the Holo- 
cephahds are associated with the Galeodes (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 Ichthyolog}^ and the student ought therefore 
to be told that placoid scales are hard 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 prepai ed 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 differ 
from ganoid scales in the absence of enamel, as well 
as in their less regular shapes ; and Kdlliker 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 be 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 Holocephalid.s, 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 tfansverse 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 pharyngo-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 
chevron 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 type 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 
Muller 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 Sharks (frafeocfes of Agass-iz) are Plagio- 
stomes with lateral gill-openings (fitigmata ) ; free supplemen- 
tary eyelids ; an incomplete scapulo-coracoid girdle suspended 
to the soft parts behind 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 first dorsal being over or 
behind the ventrals. This division is comprised in the 
family of Scylliida;. 
B. Second (fiCTsfon.— 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- 
CharIjEU.®; Spyhna;dje; and Priasnodontid.®. 
2. Second Subdivision — Sharks of 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 Galeida: ; ScYLLiODONTiDiB ; 
and Mustelihas. 
3. Third subdivision. — Sharks of B group with spout- 
holes but no blinkers. It embraces the families 
Lamnadda; : Odontaspid®: ; Ai.opeci.®da: ; Cestra- 
ciONTiD^ ; and Khinodoktidas. 
C. Third division. — Sharks having but one dorsal and one 
anal fin, forming one family, the Notiuanida:. 
D. 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 embraces 
the families of bPiNAClD.®; Scymnid®:; and IsQUA'n- 
NAii)®;. 
