OF THE SKULL IN SHAEKS AND SKATES. 225 



These thin valvular cartilages are of great interest to the morphologist : the second 

 and fourth are exactly repeated in the Snake tribe ; the fourth is often large in birds 

 (e. g. Rhea, Turnix, some " Picida;," and the " Passerinse " generally). The fifth, or 

 mandibular labial, which is absent in the Skate, but present in the Shark, I found 

 thirty-five years ago in the Coot, and later in the Gallinule. 



The third labial {1 3) has only to be compared with the " appendix alse nasi " of an 

 embryo mammal for the two to be immediately recognized as representing each other. 



Summary and Conclusion. 

 The skull of the Selachians may be expected to yield much instruction to the mor- 

 phologist. I shall compare the varieties of it seen in that great group with each other, 

 and then with the skull of various other types. 



A. The skulls of the Dog-fish and the Skate as compared with each other and with what 



is seen in the Selachians generally. 



The Dog-fish and the Skate are representatives of the two main divisions of the 

 Elasmobranchii, and are fairly typical of the two suborders. 



In their outer form the Sharks differ but little from that of an embryo Teleostean 

 at about the time of hatching ; the Skates mask this form by their extreme flatness 

 and the huge expanse of their outspread pectoral fins. Their embryos differ but little in 

 outward appearance, as Mr. Balfour's figures (Q, pi. 25) and mine clearly show ; but as 

 soon as the skeletal elements can be traced their divergent development can be seen. 



Besides the long external branchials of the hyoid and branchial arches, each type 

 shows/oiwfree external branchiae emerging from the spiracle; and the pajjillce actually 

 developed are twice that number. 



The postoral arches are, noi-mally, seven ; but there are eight in Hexanchus and nine 

 in Heptanchus. 



One important difierence has been shown in this, namely that the postoral " extra- 

 viscerals," which are so well developed in the Dog-fish, do not appear in the Skate. 



Another contrast lies in this, namely that the hyoid rays are more or less split or 

 digitate in the Sharks and undivided in the Skates. 



As a rule, the cartilage developed in the spiracular operculum in the Shark (it is 

 small in Scyllium canicula) is a ray (or rays) ; in the Skate it is part of the body of the 

 arch. There is one ray in Squatina, Mustelus, and Galeus ; two in Scymnus, and three 

 in Centrophoriis (Gegenbaur, pis. 11 & 12). 



In the Torpedo (ibid. pi. 13. fig. 3, kr, a.b) there are three small subsidiary cartilages 

 besides the main " metapterygoid " segment in front of the spiracle. In Cestracion 

 (Huxley, 4, p. 42, fig. 8, ot.p) the spiracular cartilage is a free " otic xirocess," or meta- 

 pterygoid. The transverse position of the mouth, which is so perfect in the Skates, is 

 much more oblique and projecting in the Sharks, and is more like what is seen in 



