106 Edward Phelps Allis jr., 



DoHEN, in his later work, identifies this branch as the external 

 carotid. It will be described in connection with Fig-. 2. 



In the mandibular arch there is but a single continuous vessel 

 connected, as just above stated, by commissure with the efferent 

 hyoidean artery. Ventral to that commissure, the mandibular aortic 

 arch is called by Dohrn the thyreo-spiracularis artery, and it is 

 said (Dohrn, 1885, p. 5) to arise from the anterior wall of the 

 afferent hyoidean artery, close to the point of origin of the latter 

 artery from the anterior end of the conus arteriosus. It is, accord- 

 inglj', evident that a certain ventral portion of the thyreo-spiracularis 

 must represent that section of the primary ventral longitudinal 

 vessel that lay between the hyoidean and mandibular arches; audit 

 is so shown in my diagram. Dorsal to the commissural vessel, and 

 as far upward as the mandibular pseudobranch, the mandibular aortic 

 arch is called by Dohen the arteria spiracularis. Dorsal to the pseudo- 

 branch it is called the cai-otis interna anterior. From the thyreo- 

 spiracularis, before it joins the arteria spiracularis (Dohrn, 1885), 

 or from the point where it joins that artery (Dohen, 1890), a branch 

 is said to arise, and it is called by Dohrn the arteria mandibularis; 

 but no bi'anch is described or shown as arising from the anterior 

 carotid. Such a branch is however described by Dohen (1890) in 

 somewhat older embryos of Mustelus, and is there called the arteria 

 ophthalmica magna: and as this latter artery is also shown by 

 Carazzi (1905j in the adult of Scyllium catuhis, it is quite probable 

 that it was simply overlooked by Dohrn in his embryos of Fristiurus 

 and ScijUium canicula. As it is not given by Dohrn, I show it in 

 dotted lines in my diagram. 



In Fig, 2 a stage is represented somewhat more advanced than 

 that shown in Fig. 1 , the diagram being based on Dohrn's figures 

 of Fristiurus and Scyllium, on his descriptions of later stages of 

 those same fishes and of Mustelus (1890), and on Parker's (1886) 

 figures of the arteries in Mustelus antardicus. Here the hyoidean 

 and glossopharyngeal efferent arteries are shown connected by 

 dorsal commissure, and that section of the dorsal aorta that lies 

 between the dorsal ends of those same arteries is shown greatly 

 reduced in caliber. The ophthalmica magna is shown arising from 

 the efferent mandibular artery and running directly forward, in the 

 position of a dorsal commissure, to supply the choroid gland; which 

 gland DuHRN is said (Balfour, 1881, p. 265) to have considered as 

 a possible remnant of a premandibular gill. The choroid gland is 



