276 J. T. CUNNINGHAM. 



to find, if the theory be true, that these rays were originally 

 double ; but Dohrn says nothing of this difficulty, attaching the 

 greatest importance to the musculature. It has been objected to 

 Dohrn's theory by myself and Professor Carl Vogt that in Teleos- 

 tean embryos there is a prseanal median fin in addition to the prse- 

 anal paired fins ; to which Dohrn has replied that it has not been 

 proved that this fin has any musculature, and therefore it is pro- 

 bably a new development peculiar to the class in which it occurs. 



Morphology of the Mandibular and Hyoid Arches of Selachians. 



We come next to a discussion of one of the most compli- 

 cated chapters in Vertebrate morphology, the question of the 

 mandibular and hyoid arches in Selachians. We will take a 

 rapid survey of the facts as they exist according to Dohrn^s 

 investigations, and then consider the deductions he draws 

 from them. In embryos of Pristiurus, Scyllium, Mustelus, 

 Centrina, Torpedo, and Raja the conns arteriosus at its terminal 

 bifurcation forms the hyoid arteries, the arteries of the hyoid 

 arch. From each of these arteries near its origin arises 

 another artery which runs parallel to and anterior to the hyoid 

 artery. Between the bases of these two lies the thyroid gland, 

 and the arteries are to be called the thyroid arteries. The 

 hyoid artery supplies only one series of branchial laminae, the 

 posterior. There is also but one branchial hyoid vein, the 

 posterior. There is only one venous commissure from the 

 hyoid vein instead of two as in the posterior arches, and 

 this commissure opens into the thyroid artery. The art. 

 thyroidea has hitherto been called the art. mandibularis. 

 The thyroid artery, after receiving the venous commissure, is 

 continued into the spiracular artery. The hyoid vein divides 

 dorsally into two branches, one of which runs back and joins 

 the dorsal aorta system, the other runs forward as the carotis 

 posterior, joins for a short distance behind the hypophysis 

 with the same vein of the other side, then separates running 

 one each side of the hypophysis, the vein of each side receiving 

 a large vein from the spiracular gill. 



