287 



breaks up in the muscles and tissues of the region, its terminal 

 branches meeting and crossing those of the afferent hyoidean artery, 

 already described. 



This hyo-opercularis artery of Polyodon is thus quite unquestionably 

 the homologue of all or a part of the similarly named artery in my 

 descriptions of Amia (1900) and Lepidosteus (1908), notwithstanding 

 the fact that it has its origin from the external carotid instead of 

 from the common carotid. It must accordingly, as in those fishes, 

 be the persisting dorsal portion of the primary hyoidean aortic arch, 

 or of the efferent hyoidean artery, whichever it may be, this portion 

 of this artery thus being identified in the descriptions of the adults 

 of all the ganoids I have so far considered excepting only Acipenser. 

 That Acipenser should be exceptional in this particular respect seeming 

 to require some explanation I have been led to reconsider my diagram 

 (Allis, 1908) of the cranial arteries in the adult of that fish, and 

 particularly to compare Viechow's (1891) descriptions of the arteries 

 in the adult with Ostroumoff's (1907) descriptions of the arteries 

 in embryos. This work by Ostkoumoff, although published in Dec. 

 1907, did not come to my notice until after my diagram of the arteries 

 in the adult Acipenser, above referred to, had been sent to press. 



According to Ostroumoff, the dorsal ends of the mandibular 

 aortic arches of opposite sides are, in early embryos of Acipenser 

 ruthenus, united in the middle line by a wide anastomosis, which, 

 immediately before the hatching of the embryo, becomes obliterated. 

 At about this same stage the mandibular and hyoidean aortic arches 

 become connected by a short anastomotic vessel which persists in 

 later stages and there becomes a commissure which, according to the 

 description, must connect the mandibular aortic arch ventral to the 

 mandibular (spiracular) hemibranch with the efferent hyoidean artery 

 (arteria efferens opercularis) ventral to, or opposite, the hyoidean 

 hemibranch. 



Toward the end of the first week of postembryonic life the 

 ventral end of the mandibular aortic arch loses its connection with 

 the truncus arteriosus and acquires a connection with the ventral ends 

 of both the efferent hyoidean artery (A. efferens opercularis), which 

 is prolonged ventrally beyond the hyoidean hemibranch, and the efferent 

 artery of the first branchial arch. At this same period the external 

 carotid (A. facialis s. carotis posterior externa) is found arising, as a 

 large branch, from the efferent hyoidean artery, close to the point 

 where that artery joins the lateral dorsal aorta (A. carotis interna). 

 That part of the efferent hyoidean artery that lies ventral to this 

 external carotid branch, between it and the hyoidean hemibranch, is 



