OF THE SKULL IN SHARKS AND SKATES. 227 
arches are bent forwards above, and they both simply become segmented at the bowed 
part into an epi- and ceratopleural element. 
In the first arch the epipleural element is the pterygo-quadrate, and the cerato- 
pleural the free mandible; in the hyoid the epipleural is the hyomandibular, and the 
ceratopleural the free hyoid cornu (Pl. XXXIV. fig. 1, Pl. XX XVIII. fig. 2). 
This is a very simple piece of morphology; and if the modification of these arches 
had stopped here their meaning would have been evident. 
But in the Skate we have at once a hyoid arch as difficult of interpretation as that 
seen in so many higher types of Ichthyopsida, and of the air-breathing Vertebrata 
generally ; so that the first, as it were, of a whole series of puzzles is set before us. 
In the Shark the first and second arches are merely “ branchials,” without distinct 
pharyngo- or hypopleural elements. 
In the Skate the hyoid arch abutting above against the skull does not grow over the 
pharyngeal roof, as in the succeeding arches (Pl. XXXV. fig. 4, him, hy), and has there- 
fore no pharyngo-pleural element. 
But in the forked expansion formed by the primary bar, cartilage commences in two 
places—a little nucleus in the front fork, and the apex of the main bar in the hind 
fork: thus the cartilage of the epipleural region is primarily double. A similar 
puzzle offers itself in the first arch; but the order is inverted: in it the hind fork has 
its own little nodule of cartilage, and the main bar runs in the front fork; thus the 
epipleural region of the mandible has two sources of cartilage—a small hinder part, 
and a large (the main) front part. 
In the Dog-fish the two hyoid elements get close within the articular region of the 
mandible, and are strongly strapped to it by a hyo-suspensorial or symplectic, and a 
-mandibulo-hyal ligament. 
In the Skate the little front cartilage of the hyoid arch becomes the large hyo- 
mandibular, being loosely connected with the rest of its own arch, and having the whole 
mandibular apparatus suspended to its distal end, so that the mandibular arch is 
“hyostylic,” as in the Shark; yet it is not only suspended on the upper element, but 
has also a feeble metapterygoid or otic suspension ; it is somewhat “ amphistylic.” 
B. The Skull of the Dog-fish and Skate, as compared with that of the more generalized 
Selachians, of the Chimeroids, of the Dipnoi, and of the Amphibia. 
All these types may profitably be compared together; much, however, of this work 
I find admirably done to my hand, in Professor Huxley’s paper on Ceratodus (P. Z. S. 
Jan. 4, 1876"). 
* The harmony between the author of that paper and the writer of this is almost perfect. One Jittle and 
one great difference of opinion exist: namely, his “‘angulare” in Ceratodus and the Amphibia is my “ arti- 
ticulare,” p. 34; and the second, or great difference, is the relegation of the trabeculz by him to the “ pleural” 
elements (p. 32). I haye shown in the present paper my own change of opinion, and now consider them to 
be pro-parachordal tracts, ending in exogenous “‘ pleurals.” 
