1906.] NERVES OF CHLAMYDOSELACHUS ANGUINEUS. 983 



fi'oni the pre-ti'ematic in the usual manner. The post-trematic 

 passes along the sixth arch, sending out no branches, but unites, 

 halfway along the arch, with a fine nerve (PI. LXYIII. fig. 1, Y. 6), 

 probably Vagus 6. This combined nerve (fig, 1, Y. 5 + Y. 6) 

 could be traced along the arch as far as the middle of the cerato- 

 branchial cartilage, where it divides into a number of small 

 branches which supply the arch and muscles attached to it. No 

 fibi'es have been traced on to the A^estigial seventh arch. On one 

 side, not only do post-ti-ematic 5 and Yagus 6 unite corajoletely, but 

 they are also connected by a transverse branch (fig. 1, Con. Y. 5). 



(g) When the nerve called Vagus 6 is traced upward, it is 

 found to originate from the vagal riband, between the origin of 

 Yagus 5 and the point whei-e the intestinal trunk divides. Traced 

 from its central connections Yagus 6 passes obliquely downward 

 and caudad, to fork over the anterior cardinal. The anterior pait 

 unites with the post-ti-ematic of Yagus 5 and the posterior with 

 one of the fii-st spinal nerves. The origin, course, and distribution 

 of this nei've strongly suggests that it is the remnant of the sixth 

 branchial nerve to the degenei-ate seventh branchial ai'ch. It is 

 highly probable that serial sections would show that the combined 

 nei've sends branches not only to the sixth arch but also to the 

 vestigial seventh arch, which lies in close proximity to the former. 

 The posterior division of Yagus 6 is very small and unites with a 

 spinal nerve (or nerves), whose main distribution is to the median 

 mandibular muscles. 



The anterior cardinal vein in Chlami/doselac/ms lies in the 

 position of the vanished seventh cleft, but in most Elasmobranchs 

 in the position of the missing sixth cleft. In the latter, the vein 

 sometimes lies i_n the notch, so common on the fifth branchial 

 arch, which in many cases is undoubtedly produced by a remnant 

 of the sixth arch coalescing with the fifth. Probably as an arch 

 and its cleft disappeared the anterior cardinal vein was pressed 

 forward and took up the position of the missing cleft. According 

 to this reasoning, Yagus 6 may be said to theoretically fork above 

 the seventh cleft. 



In the formalin specimen, which was a mature female, no ti'ace 

 of the seventh arch was found and Yagus 6 was in a much reduced 

 condition. Yagus 5 divided above the sixth cleft into pre- and 

 post-trematics, which were of almost equal size. The pre-trematic 

 5 had the usual course. The post-trematic passed along the 

 anterior side of the sixth arch for a short distance, then divided 

 into two branches, a small anterior and a larger ]30sterior. The 

 former continued to course along the anterior side of the sixth 

 arch and must be regarded as the true post-trematic of Yagus 5, 

 whilst the latter passed along the posterior side of the last arch 

 to be distributed chiefly to muscles attached to the unusually 

 enlarged end of the cei'ato-branchial cartilage. This innervation 

 coincides with that of the nerve described as Yagus 6 in the 

 other specimen. Hence this branch is to be regarded as the 

 remnant of Yagus 6. 



Proc. Zool. See— 1906, No. LXY. 65 



