304 
CARTILAGINEI. — SQUALID 
some or other of which are scattered over all 
seas. Fifteen are enumerated among British 
Fishes, either as habitual residents in our waters, 
or as accidental stragglers. Considerable diffi- 
culty is felt by naturalists in the attempt to 
subdivide the family into natural groups ; not 
from the number of the species so much as from 
the diversities of form and structure that exist 
among them. The following is the arrangement 
proposed by Mr. Swainson. 
HAMMER SHARK AND SAW FISH. 
1 . Zygcenina, The Hammer-heads. These have 
