MUSEUM OF COMPARATIVE ZOOLOGY. 215 



the foramen just mentioned [see Fig. 12] at the point x in Fig. 12, it 

 gives oflf a very slender vessel, y, which passes backwards and inwards 

 along the ventral aspect of the skull and vertebral column, and joins 

 with its fellow of the opposite side to form a delicate longitudinal 

 median trunk, z, which is continued backwards to the junction of the 

 first pair of epibranchial arteries. I think that there can be no doubt 

 that the posterior carotid artery, from its origin to the point x, to- 

 gether with its backward continuation, y, represents the dorsal portion 

 of the hyoidean aortic arch, or hyoidean epibranchial artery, the altered 

 direction of the vessel being accounted for by the changed position of 

 the hyoid arch. The middle trunk, z, is as obviously the actual an- 

 terior portion of what may be called the interhyoidean section of the 

 dorsal aorta. It has clearly nothing to do with the arteria verte- 

 bralis impar of Myxinoids, which it resembles at first sight, since the 

 latter is a secondary forward prolongation of the aorta altogether cepha- 

 lad of the gills. As this anterior portion of the dorsal aorta undergoes 

 complete atrophy — if indeed it ever exists — in the Rays as well as in 

 the Holocephali, it is a matter of some interest to find it persisting in a 

 typical Selachian, and one is led to inquire whether it is actually absent 

 in those two forms the arteries of which have been described, or whether 

 it has hitherto been overlooked. I can only say that I have failed to find 

 any mention of it." Parker does not give the title of Hyrtl's ^ important 

 paper in his list of literature, and makes no referetice to it anywhere in 

 his text. Presumably, then, he had at least no knowledge of its con- 

 tents, or he would certainly have greatly modified the paragraph just 

 quoted. In Chlamydoselachus, the arteries described by Parker for Mus- 

 telus are present as a strong pair of vessels diverging from the anterior 

 end of the vertebral portion of the precardiac aorta, curving outward 

 until they reach the internal carotid trunk, into which they open, some 

 distance behind the internal carotid foramina. The fusion takes place 

 even before the internal carotids begin to curve inwards toward the 

 median line. Parker's conclusion, that the posterior carotid to the 

 point X and the small vessel y form the hyoidean epibranchial artery 

 is clearly untenable when applied to the more primitive Chlamydose- 

 lachus. 



Parker's argument, that the unpaired aorta formed by the confluence 

 of the vessels y is not comparable with the arteria vertebralis impar 

 of Myxinoids as described by Miiller, is I think insufficient, since we 

 know nothing of its developmental history to enlighten us as to its 

 origin and manner of growth, and the adult condition of the vessel cer- 



