OF THE SKULL IN SHAEKS 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 (PI. XXXIV. fig. 1, PI. XXXVIII. 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 difiicult 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 (PI. XXXV. fig. 4, hm, hy), and has there- 

 fore no phai-yngo-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 ofiers 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 symjplectic, 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 Chimceroids, 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 little and 

 one great difference of opinion exist : namely, his " angidare " in Ceratodus and the Amphibia is my " arti- 

 ticulare," p. 34 ; and the second, or great difference, is the relegation of the trabeoulse by him to the " pleural " 

 elements (p. 32). I have shown in the present paper my own change of opinion, and now consider them to 

 \is pro-parachordal tracts, ending in exogenous "pleurals." 



