DEVELOPMENT OF THE SKULL IN STURGEONS. 
155 
exactly like it, but gently bent in the opposite direction; its teeth look a little forwards 
and fit in between those of the upper jaw. 
The palatine (Plate 14, fig. 5 ,pa.) is a styloid tract of bone binding on the external 
edge of the broad fore part of the suspensorium (pg.q .); it is edentulous. The ptery¬ 
goid (fig. 5, pg.) carries teeth; it is a thin plate of bone, which lies inside the hinder 
convex edge of the cartilage. 
Over the rudimentary “ orbitar process” of the suspensorium a small scale of 
bone is visible, just as when the squamosal is formed over the quadrate in the Am¬ 
phibia ;* this is the preopercular (p.op.). In the roof and sides of the pharynx inside 
the first branchial arch there are three pairs of small, arcuate, dentigerous scales of 
bone (Plate 13, fig. 12, and Plate 14, fig. 5); these are “ upper pharyngeal bones.” 
Clamping the hinder part of the hyomandibular, its rudimentary “ opercular process,” 
the opercular bone (op.) is seen ; it is a convex shell, with spurs in front and a sharp 
round margin behind. These bones are evidently all mere parostoses ; the “ectosteal” 
sheaths of the visceral arches have not appeared yet; the cranium never acquires any 
well-grafted bony plates. 
Third Stage (continued).— Horizontal sections of one of the lesser larvae of Acipenser 
ruthenus, at this stage —13-| millims. long. 
Section 1.—The uppermost of these sections (Plate 13, fig. 10) is through the hemi¬ 
spheres and the base of the mid-brain directly over the thalamencephalon (CA, C 1 .), 
the eye-balls (e.), and the front and sides of the mouth (m.). The quadrate and 
pterygoid regions (q., pg.q.) are severed, being cut through where there is a depression 
above ; moreover, the broad pterygoid plate is obliquely placed, and thus this hori¬ 
zontal section shows its thickness, but not its breadth. Between the hooked fore ends 
of these cartilages a newer tract, the lozenge-shaped azygous metapterygoid (figs. 10 
and 11, mt.pg'f is seen. Behind the transversely oval quadrate region the lower part 
of the hyomandibular ( hm .) is cut across ; it is obliquely placed, and between the two 
sections of cartilage, on the outside, the lower part of the first cleft or spiracle (cl 1 .) is 
laid open. Part of the “ adductor mandibulse” muscle (ad.m.) is seen outside the 
pterygo-quadrate bar, and the opercular fold of the first cleft is seen with its rough 
inner surface. 
Section 2.—The next section, below the last, is through the whole length of the 
pterygo-quadrate bar (Plate 14, fig. 2, pg.q.), which is seen to be of almost uniform 
thickness, and of a sigmoid shape; the fore end is turned inwards, and the hind part 
outwards. Behind it the top of the articular region of the transverse mandible (mk.) 
was sliced off. The basis cranii (tr.) is cut through close in front of the post-pituitary 
wall and the notochord, where the trabeculae are completely confluent; the large infun¬ 
dibulum (inf) is severed at its “neck” (see Plate 14, fig. 1, inf). Here, this process of 
* The squamosal of the Amphibia has a large descending 'preopercular process; but there is no separate 
preopercular bone, such as Fishes possess. 
x 2 
