58 
seems much the more probable one, the dermal component of the 
squamosal, in that case, being represented in certain of the little 
scales that lodged or protected the canal, and that are not described 
by GEGENBAUR. 
In Polyodon, CoLLInGE (No. 12) shows no squamosal bone, the 
region where that bone is usually found being occupied by what he 
calls a posttemporal and a dermosphenotic. The dermosphenotic is 
said to be almost hidden by the development, upon its surface, of a 
“series of much expanded canal bones”. Later, these canal bones are 
said to lie upon the “dorsal surface” of the dermosphenotic. Whether, 
in these statements, the canal bones are to be considered as entirely 
separate from the underlying bone, or as partly or entirely fused with 
it, it is difficult to judge. The name dermo-sphenotic is said to have 
been given to the underlying bone by BRIDGE, in a work that I have 
not at my disposal, and superficial to the bone the main lateral canal 
is said by CoLLINGE to divide into supraorbital, suborbital and hyoman- 
dibular branches. The bone thus occupies a position, relative to the 
lateral canals, that would correspond to the united dermal postfrontal 
and squamosal bones of Amia and Teleosts. 
In Accipenser the squamosal is said by Huxuey (No. 19) to be a 
“membrane bone”, and it is traversed, according to VAN WIJHE (No. 36), 
by the main lateral canal of the head. 
In the Stegocephali a squamosal, properly so-called according to 
Gaupp (No. 16, p. 105), is found, and it is said to be traversed, in 
certain species, by a lateral canal (No. 15). This canal, in Tremato- 
saurus, forms one section of a nearly circular canal which begins and 
ends at the external opening of the ear, having traversed successively 
the so-called squamosal, postorbital, jugal, and supratemporal bones. 
Whether it traverses also the epiotic, or simply lies along its lateral 
edge, I can not judge from the figures. These bones are all said to 
be dermal bones, and the canal that is said to traverse them is de- 
scribed by Frirscu (No. 15, Bd. I, p. 35) as a half cylindrical gutter 
on the external surface of the bone. The membranous sensory canal 
thus probably lay superficial to the bones it is said to traverse, but 
whether it was enclosed in separate and independent bony scales, as 
in Polyodon and certain Teleosts, or not, can not be judged from the 
descriptions, and I shall have occasion to refer to it again. 
In Gymnarchus niloticus, and in Mormyrus, conditions are found 
that seem to approach, somewhat, the conditions described in the 
Stegocephali. In these fishes, according to ERDL (No. 14), the squa- 
mosal is a semicircular bone bounding the “äußere Gehöröffnung”, 
the open part of the semicircle directed upward and backward, and 
