CHAPTEE IV. 

 SHARKS AND KAYS, 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 

 hyomandibular 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 vertebrse 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 



