350 CHORDATA. 
bones which thus belong to the visceral arches. In con- 
nection with these visceral arches are the premaxilie and 
maxille forming the upper jaws and carrying a single row 
of small teeth. From the hind-end of the maxilla is a small 
bone (the guadratojugal) which unites behind with the 
quadrate. 
In connection with the suborbital bar are paired fa/a- 
tines anteriorly and paired pzerygoids of a triradiate shape. 
On the dorsal side a pair of T-shaped sgwamosals overlie the 
quadrate cartilages. A rod-like cartilage runs from the tym- 
panum to the auditory capsule; it is called the columella 
and probably corresponds to the hyomandibular of the skate. 
The lower jaw, as in the skate, is formed of the mandibular 
cartilage, but it has also three bones. It bears no teeth. 
A large plate of cartilage, the Zngual plate (or hyoid 
cartilage), rests below the tongue ; it has two long anterior 
cornua, which are attached to the auditory capsule, and the 
posterior cornua which are shorter. It is the Ayoid cartilage, 
with perhaps a single pair of branchial arches (fosterior 
cornua). 
At first sight there is little in common between this skull and that of 
the skate. If, however, we carefully follow the following modifications 
which have probably taken place, the comparison is easier. Let us 
suppose that the palatoquadrate cartilages of the skate become fused on 
to the nasal region anteriorly and to the auditory region posteriorly, and 
that further these cartilages are bent out laterally so that they lie no 
longer under the cranium but round the outer border of the eyes. A 
condition is thus produced closely similar to the cartilaginous cranium 
of the frog. The cartilage is then replaced by bone in parts, producing 
the CARTILAGE-BONES, sphenethmoid, prootics and exoccipitals. Lastly, 
this skull is covered up by a number of MEMBRANE-BONES, paired /ronto- 
parietals and nasals above, parasphenoid and vomers below, and a 
number of others, palatines, pterygoids,.squamosals, quadratojugals, 
premaxille and maxzlle, in connection with the visceral arches. 
The vertebral column in the frog consists of nine free 
vertebree and a uvostyle. The vertebre are ossified and 
form rings. The first or atlas is a simple ring with two 
facets for articulation with the skull. The second to seventh 
are proccelous, ze., they articulate with each other by a 
concavity in front and a convexity behind. The main 
portion of the vertebra is called a centrum and above this is 
a neural arch covering over the spinal cord. A large lateral 
process on each side consists of a ¢vansverse process which 
