i6o A HISTORY OF FISHES 



{Scyliorhinus) is at once most suitable and easily obtainable. 

 It may be objected that the skull of the Lamprey {Petromyzon) is J 

 e\cn more primitive, but, although this is true of many features, ^ 

 there are others in which it has attained to a marked degree of 

 speciaHsation along lines peculiar to this class of fishes. In 

 order to reveal the skull of the Dog-fish it is necessary to cut 

 away the skin and muscles of the head, and in so doing it is 

 quite easy to cut into and damage the underlying cranium. 

 This fact should serve to fix in the mind the most important 

 feature of the skull, namely, that it is composed, not of bone, but of 

 a much softer substance called cartilage. In all living Selachians 

 the entire skeleton is cartilaginous, and this provides one of the 

 principal characters separating them from the Bony Fishes. 

 In some Sharks and Rays the cartilage is strengthened by the 

 addition of calcareous or limy matter, but true bone is never 

 developed. 



The cranium of the Dog-fish (Fig. 46A) has the form of a 

 somewhat flattened oblong box, with more or less complete 

 floor, roof, and lateral walls, but open in front and behind. 

 Through the posterior aperture the spinal cord emerges from 

 the brain, and on the cartilages forming the lower edge of this 

 opening are two prominences or condyles by means of which 

 the cranium is articulated to the first segment of the backbone. 

 Within the trough-like cranium lies the brain, and the various 

 nerves as well as the associated blood-vessels pass outwards 

 through a number of holes or foramina in its floor and walls. 

 On the outside of the box are two pairs of prominences, hollow 

 capsules attached to the cranium : the pair at the front end are 

 open below and lodge the delicate organs of smell, and the 

 other pair at the hinder end enclose the organs of hearing. 

 Between them, on either side in the centre of the cranium, is a 

 cavernous recess known as the orbit for the lodgment of the 

 eyes. So much for the skull of the Dog-fish. 



In the Bony Fishes there is a much more complex skull, in 

 which true bones have to a greater or lesser extent replaced 

 the primitive cartilage, although in some of the more generaHsed 

 forms large areas of the softer substance still remain. In the 

 Sturgeon (Acipenser), for example, the head is covered with a 

 dense bony armour made up of a large number of separate 

 and symmetrically arranged plates, but below these is a 

 cartilaginous cranium not very unlike that of the Dog-fish. In 

 the Bichir {Polypterus) , another primitive form, there is still 

 more bony matter in the skull, for, in addition to the investing 



