16 
larger than the right (cp. description of the hyomandibu- 
lar), in contradistinction to the hyomandibular cup which 
is smaller on this side. The pterotic forms the anterior 
boundary of the glosso-pharyngeal foramen as shown in 
fig. 2 (f.gl.). It is bounded by the parietal, sphenotic, 
prootic, opisthotic and exoccipital. 
Opisthotic or Intercalar (Op.0., figs. 2, 3, 4).—Forms 
the remainder of the parotic process. It is by far the 
smallest of the otic bones, consists of a thin flat plate of 
irregular shape, and contains no cartilage whatever. Its 
development should therefore be studied. It forms the 
posterior boundary of the glosso-pharyngeal foramen (f.9/., 
figs. 2, 3), and is bounded by the basioccipital, pterotic 
and exoccipital. 
There can be no question in forms such as the Cod 
and Plaice that the ear bones described as pterotic and 
sphenotic are something more than what they seem, ~., 
they have a compound and not a simple origin. Added to 
the so-called cartilage bone in each case is a dermal 
element, originally developed around that part of the 
sensory canal system associated with these bones, and now 
more or less completely fused on to them. In some Fishes 
(such as the sphenotic of Amia) the two portions remain 
distinct throughout life, and in others the line of fusion 
may be plainly seen, with, however, the bones remaining 
separate as an occasional abnormality (such as the pterotic 
of the Cod). But as a rule the two portions fuse com- 
pletely, so as to be indistinguishable in the adult. Now 
in the one case the terms pterotic and squamosal have 
been applied indifferently to the compound of the adult. 
We may therefore, in those cases where the two parts of 
the compound remain separate in the adult, call the car- 
tilage pterotic, or ear bone, the true pterotic, and the 
dermal pterotic, or lateral line bone, the squamosal. In 
