OF THE SKULL IN SHARKS AND SKATES, 221 
branchial rays proceeds from each; these are pedate at their outer ends. The ventral 
segment (hypobranchial) is more or less adze-shaped (fig. 5, h.dr); but this obtains 
only in the second to the fourth. The first has this segment very long, at first dilated, 
and then very slender, and the right and left are early fused together. The last 
branchial arch has its pharyngo-branchial united to that of the fourth, and its hypo- 
branchials (h.d7. 5) completely united. These fused elements look like an azygous 
piece; but in the adult of 2. clavata (Pl. XLII. fig. 4, h.dr 5) they are very partially 
united, and, as to form, are seen to be only a modification of the adze-shaped or fan- 
shaped type. 
At this stage the branchial arches, including the hyoid, carry two sets of branchiz 
in full function. The eaternal are at their highest development ; and the internal plates 
are perfect, although small. 
The “extrabranchials” are absent, as far as I can make out, in different kinds of 
Skates’ (2. clavata, R. maculata, &c.); nor is there a labial on the mandible. But there 
are four pairs of preoral labials—three acting as nasal valves (/ 2, 3, 4), and the pair 
(/1) attached to the side of the “rostrum,” but not riding upon the nasal sac, as in the 
Dog-fish. 
Third Stage: Embryos of the Thornback Skate (Raia clavata) nearly ready for exclusion 
JSrom the egg-pouch. 
In the last stage the metamorphosis was complete; this third stage is given for the 
sake of the vertical and transverse sections, which reveal the architecture of this kind of 
chondrocranium. A longitudinally vertical section with the brain removed (Pl. XLI. 
fig. 1) shows a hollow barge-like structure, with a cartilaginous bottom perfect, and 
projecting as a free prow—the basitrabecular rostrum (0.t7)—whilst the “deck” is only 
cartilaginous fore and aft over the ethmoidal and the auditory region. The various 
nerye-outlets (1, 2, 5, 7, 8, 9, 10) are easily recognized. This little boat is undergirded 
by transverse bars that appear in the section—the pterygoid, mandibular, and first and 
fifth hypobranchials (¢.pg, mn, h.br 1, h.br 5). 
The first transverse section shows the nasal caps or domes (o/) far apart; these are 
connected by a large bridge of cartilage, convex below and concave above. Thus there 
is formed a large precranial space ; for the frontal skin is convex. 
An inner labial (J 2) is seen in section ; the palatal skin follows the convexity of the car- 
tilaginous bridge; that bridge is formed by the trabecule and their commissure (¢7.cm). 
Here are the very elements of the nasal septum and roof of the higher types; but the 
trabecule have only united below; they are far apart; and their crest applies itself 
normally to the inner edge of the nasal dome, yet forms a structure widely different 
from that which obtains when these domes are closely adpressed, and the trabecular 
‘ In the Torpedo the dilated ends of the branchial rays unite outside the pouches in such a manner as to form 
a practical “ extrabranchial” band (Gegenbaur, pl. 13. fig. 3, pl. 20. fig. 1). 
vou. x.—part iv. No. 5.—WMarch 1st, 1878, 21 
