2S Scientific Proceedings, Royal Dublin Society. 



cartilaginous rays are also wanting in the mandibular arch, the 

 doctrine which represents the upper and lower jaw as one visceral 

 arch, homodyDamous with the branchial arches, becomes consider- 

 ably shaken, to say the least of it. 



The development of the cartilaginous parts of the hyoid arch 

 in the rays differs from that in the sharks. We find two cartilage- 

 centres in the arch — one close to the posterior margin, the other 

 near the spiracle. "We equally have two muscular centres in 

 connexion with the two cartilaginous centres. The first cartilage 

 bears cartilaginous rays, four of which are dorsal, and four ventral 

 of the venous commissure. The latter divides the arch into a 

 dorsal and a ventral part, as in other arches. "We have, therefore, 

 in this arch a complete system of muscles, except the adductors 

 and musculi interarcuales dorsalis. "We further have an artery, a 

 posterior vein, and a posterior complete row of branchial lamellae. 

 Only the anterior row of lamellae and the anterior vein are wanting, 

 while the venous commissure is present. But, nevertheless, accord- 

 ing to the current opinion, especially held by Gregenbaur, this arch 

 is only said to correspond to the ventral portion of the hyoid arch, 

 while the dorsal — the hyomandibular — has been differentiated 

 from it. 



The second cartilage mentioned above represents the hyoman- 

 dibular. It possesses its own strong muscular system which, how- 

 ever, lies only dorsally. If the fact is taken into consideration, 

 says Dohrn, that in the rays we have two cartilaginous centres, 

 and two muscular systems in the hyoid arch, we must come to the 

 conclusion that we are dealing here with two segments united into 

 one visceral arch. The posterior segment is represented by the 

 true hyoid arch. Dohrn concludes from this, contrary to Gregen- 

 baur, that in the rays the hyoid arch is in a more primitive condition 

 than in the sharks. According to Gregenbaur, the hyomandibular 

 in the rays represents only the mandibular process of the hyoman- 

 dibular in the sharks ; however, the former possess cartilaginous 

 rays in the dorsal part of the hyoid in considerable number, dorsal 

 to the venous commissure, which fact contradicts Gregenbaur's state- 

 ment. 



The spiracular cartilage in the elasmobranchs used for investiga- 

 tion is originally always a single cartilage. It is invariably situated 

 in front of the spiracular system of blood-vessels. In this respect it 



