486 



the ventro-lateral edge of the chondrocranium, it is evident that 

 there is much to favor the assumption that they are all vertebral 

 arteries and that the anterior one has secondarily and exceptionally 

 acquired a connection with the hyoideo-glossopharyngeus dorsal 

 commissure. But there is also much to favor the assumption that 

 it is an efferent visceral artery, for the little branch arising from it 

 may be simply a nutrient artery such as is frequently found arising 

 from certain of the epibranchial arteries in fishes. And it is not to 

 be overlooked, in this connection, that Dohrn has suggested the 

 possibility of the posterior carotid of his nomenclature being devel- 

 oped from the anterior efferent artery of the hyoidean arch and not 

 from the main epibranchial portion of the primary efferent artery of 

 that arch. 



What Dohrn says regarding this is as follows (Dohrn, 1890 p. 400) : 

 „Wäre man gezwungen, diese Commissur [hyoideo-glossopharyngeus 

 dorsal commissure] anders zu deuten, so erwüchse eine fast unübersteig- 

 liche Schwierigkeit für die Deutung des Stückes der Carotis posterior, 

 welches von der hinteren Hyoidvene bis zur Einmündung des ersten 

 Aortenbogens reicht. Dieses Gefäß müßte dann als Eest einer vor- 

 deren Hyoidvene aufgefaßt werden, welche zu Grunde gegangen wäre, 

 aber vor dem Zugrundegehen sich in abnormer Weise mit der hinteren 

 Hyoidvene verbunden hätte." And the conditions in Chlamydo- 

 selachus favor this interpretation of the vessels ; for until the develop- 

 ment of these vessels is known, it would seem as reasonable to 

 assume an abnormal development of the anterior efferent hyoidean 

 artery as such a development of one of the vertebral arteries. But, 

 however this may be, it seems best for the present to consider the 

 posterior carotid of Heptanchus as the true dorsal end of the efferent 

 hyoidean artery, and it is so shown in the accompanying diagrammatic 

 representation of the arteries in this fish (Fig. 2). 



The dorsal aorta of Heptanchus diminishes rapidly in cahber 

 anterior to the point where it receives the efferent arteries of the 

 first branchial arches, and as a small median vessel runs forward 

 until it reaches the ventral surface of the hind end of the skull, giving 

 off in this part of its course, on either side, a small vertebral artery, 

 this artery being the one already referred to as having its origin from 

 the aorta slightly posterior to the foramen that perforates the latero- 

 ventral edge of the skull near its hind end. Having reached the hind 

 end of the skull the median dorsal aorta separates into two parts, 



