263 



the internal carotid, and to become branches of the external carotid. 

 The retiualis artery is probably always enclosed in the skull wall, with 

 the cerebral trunk, notwithstanding that it is not always so given in 

 descriptions. 



In larvae of Amia the conditions are markedly different from 

 those in Ameiurus and the other Teleosts just above considered. In 

 that fish the internal carotid having traversed the myodome, as in the 

 Acanthopterygii, perforates the presphenoid cartilage of the base of 

 the skull, in a special canal, or foramen, and definitely enters the 

 cranial cavity; no membrane or tissue here surrounding it, as in Scor- 

 paena. There it runs forward along the side wall of the skull, giving 

 ofi" first a large cerebral branch and then the retinalis artery, which 

 latter artery perforates a membranous portion of the side wall of the 

 skull and, accompanying the opticus, enters the eye ball. The internal 

 carotid then continues forward, sending small branches to the brain 

 and finally, accompanying the olfactorius, enters the nasal sac, where 

 it is connected, by anastomosis, or commissure, with a branch of the 

 external carotid that comes from the orbit. Amia thus presents, in the 

 terminal branches of its internal carotid artery, a selachian (Parker, '86) 

 rather than a teleostean arrangement; and the same is doubtless true 

 of Lepidosteus also, for in the descriptions of the arteries of this latter 

 fish an orbito-nasal branch of the internal carotid can not be reorganized. 

 And this selachian arrangement, rather than the teleostean, resembles 

 that in the Amphibia. 



The carotid arteries of Ameiurus and the Acanthopterygii could 

 thus have been readily and directly derived from those in Polypterus, 

 while they could not have been so readily and directly derived from 

 those in Amia; and, in so far as Ameiurus is concerned, the same is 

 especially true of the myodome. Furthermore, the myodomic con- 

 ditions in the adult Ameiurus, while they could easily arise from 

 those in Polypterus, could not, it is to be especially noted, give origin 

 to the conditions found in Teleosts. For there is, in Ameiurus, no de- 

 finitely defined pituitary space, the space which is utilized to form the 

 teleostean myodome, and there also is no pituitary branch of the ju- 

 gular vein; and this pituitary vein, passing from the pituitary space 

 into the orbit, seems a necessary preexisting condition to the devel- 

 opment of a myodome. In Ameiurus the pituitary region is drained 

 by a vein that descends in the median line from a venous commissure 

 that passes from one side to the other dorsal to the hind ends of the 

 hypoaria. This same commissure is found in Scorpaena, each half of 

 the commissure apparently representing a part of the mesencephalic 



