BLUE SHARK. 35 



but the manner of formation and progress is otherwise the same. 

 In the last mentioned instances, however, the teeth are short, and 

 therefore are easily brought through the coats of the cells; but 

 this is not the case with the Blue Shark and some others. A 

 vacant space of softer texture in the rear of each tooth is to 

 them of importance, as securing to them a thinner and weaker 

 place at which they can burst through; while the contraction 

 of the fibres of the membrane, by drawing the teeth subsequently 

 together, secures to them even a greater firmness of support 

 than if there had not been an original separation. But the 

 rigidity of those contractile fibres does not stop here. Nourish- 

 ment is by this means diminished, and finally ceases. The tooth 

 becomes a dead substance, and soon falls off" with the membrane 

 itself that held it, to be j^i'esently succeeded by a new race 

 that must pass through the same changes, and to be shed again 

 in their turn. A limit is thus j)ut to the number of rows the 

 fish can be furnished with, and security taken that no old or 

 useless teeth shall remain to encumber the jaws. 



From the references given to authors who have described 

 some kind of Blue Shark, it appears highly probable that more 

 than one species exist, and may sometimes visit the British 

 coasts; although I do not feel assured of being able to lay down 

 definite marks by which they may be distinguished from each 

 other. I must content myself for the present, therefore, in 

 producing such evidence on the subject as shall serve to call 

 the attention of naturalists to further inquiry, rather than run 

 the risk of misleading them by speaking with greater certainty 

 on their specific distinctions. On comparing two specimens a 

 considerable difference is seen in the form of the head and 

 eye, as well as in the tail; which in one instance runs nearly 

 straight backward in a line with the body, while in the other 

 this organ is wider and more elevated. 



I make but little account of the variety of colour described 

 by Risso, in his Squalus Ronddetii, because it is known that 

 most fishes are liable to variation in this respect; and it is 

 especially the case when they have changed the water and bright 

 skies of the Mediterranean for the more sober tints of the 

 British Channel. But the difference of the teeth is a more 

 important distinction; more especially as I am able to affirm 

 with confidence that this character of serrated teeth is not an 



