CHAPTER IV. 
Sharks and Rays,— Subclass Elasmobranchii. 
The last subclass of the fishes is represented by the existing sharks and rays, 
together with a number of more or less closely allied extinct forms; some of the 
latter being the most primitive members of the order yet known. Indeed, taking 
these primitive types into consideration, and remembering that sharks and their 
allies are the oldest fishes with which we are acquainted—dating from the lower 
Ludlow beds of the Silurian epoch—it seems probable that the present subclass 
may have been the stock whence all other fishes were derived. Agreeing with 
the bony fishes and ganoids in having the suspending apparatus of the lower jaw 
movably articulated to the skull (generally with the intervention of a distinct 
liyomandibular element), the sharks and rays have the skeleton entirely cartila¬ 
ginous throughout life ; membrane-bones—except in one extinct group — being 
entirely wanting. The gills open by separate external clefts, and have no cover. 
When bony elements are developed in the skin, these agree in structure with teeth, 
and have nothing: to do with true bone. In all the living members of the subclass 
the optic nerves cross one another without giving off any mutually interlacing 
fibres, the arterial bulb of the heart is furnished with three valves, the intestine 
has a spiral valve, the eggs are large and detached, and an air-bladder is wanting. 
The whole of the existing representatives of the subclass form an order 
(Selachii) characterised by the cartilaginous internal skeleton being, as a general 
rule, only superficially calcified; while, except in some of the earlier extinct types, 
the notochord is constricted at the centre of each vertebra. The superior and 
inferior arches of the vertebrae are short and stout, and intercalary cartilages are 
very generally developed. The pectoral fin has not a segmented longitudinal 
central axis, its cartilaginous rays forming a fan-shaped structure radiating from 
an abbreviated base, into the anatomical details of which it will be unnecessary to 
enter here; and the axis of each pelvic fin is developed in the males into a 
“ clasper,” connected with the reproductive function. With regard to the structure 
of the skull, it may be mentioned that the hyomandibular usually intervenes 
between the palatopterygoid bar (forming the functional upper jaw, and carrying 
the teeth) and the cranium proper; but in the genus Notidanus the hyomandibular 
takes no share in the support of the jaws, the palatopterygoid bar articulating 
directly with the cranium by means of a facet behind the socket of the eye; this 
structure being probably the original one. We have already said that the tooth¬ 
bearing palatopterygoid bar serves the function of an upper jaw, by which name 
it may be conveniently referred to; and similarly the functional lower jaw is in 
reality the element known as Meckel’s cartilage. The gills are attached to the 
o c 1 
