164 
MR. W. K. PARKER OK THE STRUCTURE AND 
cartilages appear right and left of the notochord. That rod (nc.) is invested with 
a thick mesoblastic sheath of true cartilage, which, as the sections show, is thoroughly 
confluent with the parachordals in the basis cranii. In front (Plate 16, fig. 2, nc., 
20 x 1 ., py.), it runs up nearly to the pituitary space, but does not ascend into 
the post-clinoid wall—an arched and almost horizontal plate of cartilage, the true 
organic end of the skeletal axis ; all the rest, to the end of the snout, is formed 
of special outgrowths from the fore end of the basal plates. The relative size and 
thickness of the various parts of the chondrocranium will be shown afterwards in 
the description of the sections. 
C. Visceral arches of a young Sturgeon 7\ inches long. 
There is no distinct rudiment of any arch in front of the mandibular, with its 
extended and complex pterygo-quadrate “ pier;” the rudiment of the ethmo-palatine 
cartilage only exists as an extension of the aliethmoidal mass (Plate 16, fig. 1, 
al.e.). The pterygo-quadrate plate (Plate 16, figs. 1, 4, 5, 6, pg.q.) lies in an almost 
horizontal plane, and at a very variable distance from the basis cranii. The right 
and left plates meet by their extensive straight upper edge ; then they curve out¬ 
wards and backwards ; their fore margin is rounded, their inferior edge concave, 
and their hinder edge is sinuous and notched. Outside the rounded condyle ( q .) 
there is a leafy growth, imperfectly adze-shaped, which passes outside the adductor 
mandibulse; this is the “orbitar process” (Plate 16, figs. 1, 7, or.p.). Three-fourths of 
their inner face is invested by the pterygoid bone (pg.), a thickish plate with a deep 
gap in its blade, in front ; the front third of the cartilage is bare, and its hinder 
margin has suffered absorption—not direct ossification, through the pressure of the 
pterygoid bone. A sharp style of bone, with its broad end in front, lies along the 
concave antero-inferior margin ; this is the palatine (pa.) —a mere parostosis. 
A larger bone curves round the front of each plate, where the two sides meet—both 
bone and cartilage—and then runs backwards, outside the adductor mandibulee, and is 
attached to the outer face of the orbitar process; this is the maxillary (mx.). 
Mounted on the hind part of the maxillary, and at right angles with it, there is 
a little triangular bone, with its apex upwards ; it binds against the ribbed outer 
edge of the quadrate region of the cartilage; this is the pre-opercular ( p.op .), smaller, 
here, than in Lepidosteus. 
The mandible (mk) is shorter than the forwardly extended pier, it is like that of a 
Tadpole, having a thick articular region, a hollow for the quadrate, a rounded angular 
process, and a short terete main rod ; a flat dentary bone (d.) invests its outer surface, 
which has a similar outline to the rod. 
The most remarkable part of this apparatus, however, is the common compound 
(tesselated) “ metapterygoid ” region. At first (Plate 13, fig. 10) the main middle 
piece, only, was present; then a right and left segment appeared (Plate 15, fig. 1); 
now (Plate 16, figs. 1, 5, 6, mt.pg ., mt.pg".) there are fifteen. These have a general 
