226 MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
other fishes in that respect; in Hexanchus and Heptanchus (Gegenbaur, pl. 10) the 
mouth is very Batrachian. 
The upper fontanelle is more completely closed in Sharks than in Skates, although 
some of them have a nearly perfect ‘‘ tegmen” (ibid. pls. 7, 8, and 13). 
The “cutwater,” or facial rostrum,‘is least developed in the Sharks, and attains its 
highest development in Skates, especially in Pristis. 
In Sharks the nasal domes approximate in the adult ; in Skates they are permanently 
far apart (ibid. pls. 7, 8, 13, 14). 
The “ aqueduct” leading to the ear-labyrinth is seen in the roof of the skull in both 
types; and they agree in a large number of characters, as the double occipital condyles, 
&c. But their points of nonconformity are of the highest interest, and this especially 
in regard to the “ visceral arches.” 
The structure of this group seemed to me for some time to be most conclusive 
against the theory of the independence of a palatine arch in front of the mouth, as the 
pterygo-quadrate arcade is in them manifestly the foreturned upper region of the 
mandibular arch, or a huge outgrowth or process from that arch. 
But much comparative study of the Selachian skull and that of the Amphibians has 
shown me that I had been missing the true “ ethmo-palatine ” element, a very distinct 
thing from the pedate process of the quadrate or mandibular pier. 
In some Sharks, and in all the Rays, a rib-like cartilage grows in front of the eye on 
each side, either attached to the nasal dome itself or to the lateral ethmoidal region. In 
many of the Sharks it is exogenous, and does not exist in the form of a separate carti- 
lage; but it is much more clearly seen in the embryo than in the adult (Pl. XX XVII. 
figs. 1 & 3, a.o). 
It is most distinct in Heptanchus, and is very definite in Hevanchus (Gegenbaur, 
pl. 1. figs. 1 & 2,). The process can be seen in Acanthias (ibid. pl. 2. fig. 3, m’); but 
all Gegenbaur’s figures show in the Rays what I have found in Raia maculata and 
clavata—namely, a large antorbital or ethmo-palatine cartilage, whose title to be called 
a rudimentary visceral arch I shall discuss anon. 
The trabeculae, up to, or even between the nasal sacs, must be considered to be cranial 
and not facial ; yet in front they send out three facial “ processes,” that in an exogenous 
manner represent visceral arches. 
Thus it appears to me that there are visceral rudiments in the face both before and 
behind the nasal capsules. That these arrested arch-piers derive their nervous supply 
from the huge crowded nerves that also freely grow down into the postoral region, cannot 
surely tell against their ventral or visceral character; they are aborted or arrested piers, 
and have no free inferior arch, like the mandible and the hyoid cornua. 
The sharpest contrast between the Shark’s and the Skate’s facial basketwork is seen 
in the manner in which the hyoid arch becomes segmented and specialized. 
In Scyllium canicula, as we have just seen, both the primary mandibular and hyoid 
