OF THE SKULL IN SHARKS AND SKATES. 201 
suspensorium ; a complete separation of the upper from the lower part of the strongly 
bowed mandibular rod has taken place; but the pier, instead of growing upwards as a 
cartilaginous “ pedicle” to be attached to the trabecular elbow, as in the Amphibia, 
is merely fibrous upward; and the cartilage grows a far way forwards, even to the 
olfactory sac, and then turns almost directly inwards to form a junction with its fellow, 
as in the free jaw below (Pl. XXXV. fig. 7, gpg, mk). 
At present the free jaw-pieces are thick where they are scooped in their articular 
region for the quadrate condyle, but they lessen gradually to the chin; their direction 
is forwards and somewhat downwards. 
The next or hyoid arch has its sides subdivided in the same manner as the mandibular ; 
there is one transverse cleft a little above the middle, dividing the bar into an epi- and 
a cerato-hyal; here, however, the scooped face is on the upper and the rounded head on 
lower segment. 
The upper piece does not quite correspond with the hyomandibular of the Osseous 
Fish, where the primary bar is split down from top to bottom; here it is merely divided 
by a sinuous cleft. A shield-like plate, the basihyal (4.hy), unites the two lower pieces 
under the throat. The upper piece, called, ichthyotomically, the hyomandibular (/.m), 
has a broad top, which is applied closely to the inferolateral region of the ear-sac (aw). 
At its middle in front it becomes angular, and is strongly attached to the arch in front. 
The proper branchial arches have a part that runs under the hind part of the head 
and neck, that is separately chondrified ; this is the pharyngo-branchial piece (p.dr. 1) ; 
it is turned backwards and inwards. ‘There is also a division of the bar into an epi- 
and ceratobranchial (¢.0r, c.br) ; and afterwards there will be a hypobranchial piece below. 
These things will be best seen in the next stage. 
At present the four external branchial filaments growing out of the “spiracular 
opening” are one third the length of the exceedingly long clubbed threads that break 
from the second cleft (fig. 6, ¢.d7). They are all clubbed at the free end, where the 
single capillary loop turns back again to the main vascular arch. 
After the time of this stage the embryo grows much faster in bulk than in length ; 
the snout becomes like that of the adult (Balfour, /.c. pl. 25. figs. P,Q); the carti- 
laginous side walls and roof of the cranium chondrify at a rapid rate; also the facial 
rods become fully segmented and their metamorphosis fairly completed. 
Third Stage. Embryos of Dog-fish 13-2 inches long. 
The Dog-fish has fairly undergone its metamorphosis at this stage, although there 
are many important points to be noticed, in which the parts differ from the state of 
things in the adult ; this is largely a matter of relative size. 
The mesocephalic flexure is lost (Pl. XXXVI. figs. 3-6), and the ‘median trabecula” 
(fig. 6) is now a mere fissure between the medulla oblongata and the mid brain. The 
