26 Scientific Proceedings, Royal Dublin Society. 



stand, therefore, says Dohrn, that by means of a reduction of that 

 part, a coalescence of the paired fins must have taken place. 



The medullary canal must originally have been an open plate. 

 The formation of an unpaired dorsal fin may have followed the 

 closing of the plate to form a tube, the dorsal fin having most 

 probably been paired like the ventral. 



We have every reason to look upon vertebrates as segmented 

 animals ; and as we see the muscles, nerves, and the skeleton appear 

 metamerically, we may also look upon the fins as having originally 

 been metamerical. Hence we have arrived at the conclusion that 

 the ancestral vertebrate possessed two dorsal and two ventral 

 metamerically separate folds, an alimentary tract (continuous 

 through the whole body), and a medullary plate. If we compare 

 this with an annelid, the striking similarity becomes evident, the 

 dorsal and ventral parapodia in the latter corresponding to the 

 ventral and dorsal of the former respectively. 



VII. — The Origin of the Hyoid and Mandibular Arches of 

 Elasmobramhs. 1 



There is hardly a subject in animal morphology which has 

 been so much discussed as this, and to make matters worse, almost 

 every author has invented his own terminology. 



The arteries of the hyoid arches form the terminal bifurcation 

 of the conus arteriosus, just in front of the arteries belonging to the 

 first true branchial arch. The small thyroid or mandibular arteries 

 take their origin at the base of the first-mentioned arteries. The 

 hyoid arteries supply only the posterior row of branchial lamellae. 

 There is, likewise, only one hyoid vein, and consequently only 

 one commissure instead of two, as in the posterior arches, and thus 

 it sheds its blood into the commencement of the spiracular artery 

 which is the continuation of the thyroid artery. (See fig. vn.) 



The hyoid vein divides into two branches, one of which runs 

 backward joining the aortic system, while the other is directed 

 towards the hypophysial invagination, where it unites with the 

 branch from the opposite side. They soon separate again, however, 



1 Ibid., vol. vi. 1885, pp. 1-48. 



