HAYS AND SHARKS. 
1085 
EL ASMOBRAN CHII PLAGIOSTOMI. 
Elasmobranchs with several outer branchial apertures and with more or less distinct'' spiracles. Skin com- 
monly 0 shagreened, with papillse and spines (placoid scales). Notochord constricted by the formation of ver- 
tebrae. Palatoquadrate part mobile independently of the rest of the skull and dentigerous. Paired fins 
inserted horizontally. 
That course of evolution from the PaJceichtliyes 
which is represented in our times by the Plagiostoms, 
the suborder of the Sharks and Rays, has advanced so 
far, as regards the structure of the spinal column, that 
the most highly developed forms possess complete ver- 
tebrae, fully divided from each other, and retaining the 
notochord only in the intervertebral spaces between the 
centra, which are amphiccelous or conically hollowed at 
both ends. In their typical forms the two well-known 
phalanxes — the Sharks, with their elongated, fusiform 
or clavate body, and the Rays, whose pectoral fins, ex- 
panded and coalescent with the sides of the body, give 
the forepart (the head and trunk) a discoidal shape — 
are even externally so unlike each other that no other 
character than the outer shape seems necessary, espe- 
cially in the case of the Scandinavian fauna. But inter- 
mediate forms, Rays with less expanded pectoral fins 
or with an unusually terete and thick tail, and Sharks 
with triangular, wide-based pectoral fins and a body 
more than commonly depressed, occur in foreign seas; 
and during youth — in a larval stage when the Pla- 
giostoms are still furnished with filamentous external 
gills — the Rays have a great external resemblance to 
Sharks. The latter were undoubtedly the earlier forms, 
geologically speaking, and in the present age the most 
imperfectly developed forms — as Hasse in particular 
has pointed out with respect to the structure of the 
spinal column — are Sharks. 
Even from the Silurian deposits ichthyodorulites 
are known which have been referred to a Selachoid 
genus ( Onchus ), and in the Carboniferous seas Sharks 
were very numerous: — the family of the Cestraciontidce, 
which still survives in the Pacific,, already existed. And 
ever since the Jurassic Period the Not.idanoid family 
has survived with a structure which in the tropical 
and subtropical seas of the present age represents 
the lowest stages in the evolution of the Plagiosto- 
mous type. 
Individually too most of the Plagiostoms show 
great tenacity of life. During the fishery for the Green- 
land Shark in the Arctic Ocean between Norway and 
Bear Island, these large fishes are hauled up from a 
depth of a hundred fathoms or so and deposited on the 
deck of the fishing-vessel. There they lie motionless, 
partly, no doubt, owing to their sluggish temperament, 
but probably stupefied as well by the sudden reduction 
of the pressure in which they are accustomed to live. 
Their belly is opened with a large knife, and the liver 
removed for the sake of the oil which it contains; but 
nothing is done with the rest of the body, unless a 
fresh bait be required for the huge hook. A Greenland 
Shark in this condition may be - skinned and entirely 
disembowelled; but the manifestations of life do not 
cease for many hours. Even after the head has been 
cut off from the body and has lain some hours by 
itself, it is dangerous to get one’s fingers between the 
jaws, for they may easily be bitten off, and the bite is 
so tenacious that one may attempt in vain to extricate 
what the jaws have once grasped. Couch tells an anecdote 
of a Blue Shark which had been hooked and deprived 
of its liver in a similar manner. With the entrails 
hanging out of the belly it was restored to the water 
and followed the fishing-vessel for some time. It was 
not long before the fish tried to seize a Mackerel that 
had dropped from the net of the vessel. On another 
occasion a Shark was thrown overboard after being 
decapitated. For some hours the fish kept swimming 
round about — as a boy on board expressed himself — 
as if the body was looking for its head. Of the common 
Skate the same author relates that its heart may be cut 
out and retain contractile powers for at least 25 hours, 
when he saw the auricle beating five times a minute. 
° rclayiog, slanting , oblique , and Gvoya, mouth. 
b Though sometimes obliterated. 
c The Electric Rays and young Eagle-Rays, with smooth skin, are exceptions. 
