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 lat- 

 ter 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 aparatus 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 cartilaginous 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 ordor (Sel- 

 achii) characterized 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 gen- 

 erally 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 palatoptery- 

 goid 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 prob- 

 ably 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 skin by their margins, and 



(2895) 



