226 ME. W. K. PARKER ON THE STRUCTURE AM5 DEVELOPMENT 



other fishes in that respect; in Hexanchus and Heptanchus (Gegenbaur, pi. 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. pis. 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. pis. 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 Eays, 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 (PI. XXXVII. 

 figs. 1 & 3, a.o). 



It is most distinct in Ilejitanchus, and is very definite in Hexanchus (Gegenbaur, 

 pi. 1. figs. 1 & 2, m). The process can be seen in Acanthias (ibid. pi. 2. fig. 3, m') ; but 

 all Gegenbaur's figures show in the Eays what I have found in Raia macidata and 

 clavata — namely, a large antorbital or ethmo-palatine cartilage, whose title to be called 

 a rudimentary visceral arch I shall discuss anon. 



The trabeculse, 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 



