BLUE SHARK. 
35 
but tbe 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 j but 
this 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 presently succeeded by a new race 
that must pass through the same changes, and be shed again 
in their turn. A limit is thus put 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 Rondeletii, 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 
