288 



then said to become so small that the larger part of the blood from 

 the hemibranch must pass ventrally, and then forward through the 

 commissure that connects the efferent hyoidean artery with the man- 

 dibular aortic arch (A. atferens spiracularis), into the latter arch, 

 instead of dorsally into the lateral aorta. In addition to the efferent 

 hyoidean artery, which extends the full length of the hyoidean arch, 

 the ventral portion of the primary hyoidean aortic arch is shown 

 persistiDg as a short afferent opercularis artery. This afferent artery, 

 arising from the truncus arteriosus, is not mentioned in Virchow's 

 descriptions of the adult, and Virchow states definitely that in the 

 adult the afferent artery to the mandibular (spiracular) hemibranch 

 and his ramus opercularis, which is the functional afferent artery to 

 the hyoidean hemibranch, both arise from the ventral prolongation of 

 the posterior branch of the efferent artery of the first branchial arch. 

 OsTROUMOFF shows thcse two arteries both arising from a ventral 

 prolongation of the anterior branch of the efferent artery of the first 

 branchial arch, instead of from the posterior one. 



The point, however, to which I wish to call particular attention 

 in Ostroumoff's descriptions of larvae is that the external carotid 

 artery has its origin from the efferent hyoidean artery near its dorsal 

 end, for, comparing this with Virchow's descriptions of the arteries 

 in the adult, it is evident that the communicating branch said by 

 Virchow to be sent from his arteria retrohyomandibularis (my external 

 carotid) to the hyoidean hemibranch is simply that greatly diminished 

 portion of the efferent hyoidean artery of Ostroumoff's descriptions 

 that is said to lie ventral to the external carotid, between that artery 

 and the hyoidean hemibranch. The remaining portion of the efferent 

 hyoidean artery, that is its extreme dorsal portion, is then quite 

 certainly represented in that basal portion of Virchow's arteria retro- 

 hyomandibularis that lies between the point of separation of that 

 artery from the internal carotid and the point where it gives off the 

 communicating branch to the hyoidean hemibranch. Virchow, in fact, 

 himself suggests (1890, p, 588), as one of two alternatives, that the 

 A. efferens hyoidea of selachians may be represented in his A. retro- 

 hyomandibularis, this latter artery not being recognized by him as 

 the external carotid and the wording of the expression seeming to 

 indicate that he was considering the entire retrohyomandibularis and 

 not simply a short basal portion of it. 



Returning now to Polyodon, it is evident that in this fish, as in 

 Acipenser, the dorsal end of the efferent hyoidean artery is represented 

 in the basal portion of the external carotid, the latter artery being 

 simply a branch of the former. If this condition in these two fishes 



