262 



terior, seems to quite conclusively prove that it has not, in this part 

 of its course, an intracranial position. 



Comparing these conditions in Scorpaena and Belooe with those in 

 Ameiurus, it is evident that the dense fibrous tissue that sourrounds 

 the opticus as it issues from its foramen in Ameiurus must be in- 

 corporated in the skull in the other two fishes. But reference to 

 Polypterus will make the comparison clearer, for in Polypterus there 

 are myodomic conditions that are wholly wanting in Ameiurus. In 

 Polypterus, as already stated, the internal carotid has a wholly extra- 

 cranial course, and, while there is no markedly developed mass of 

 fibrous tissue surrounding the issuing opticus, as in Ameiurus, there 

 is still a certain amount of it, traversed by the cerebral branch of the 

 carotid. As a myodome was developed, from the conditions shown in 

 this fish (Allis, 1), this tissue, with the enclosed artery, would cer- 

 tainly be carried back with the opticus to the hind end of the orbit; 

 and, as the artery passes anterior to the three eye muscles that have 

 their origin at the hind edge of the opticus foramen, the artery would 

 naturally be pressed toward the middle line and would there finally 

 coalesce with its fellow of the opposite side. If then a basisphenoid 

 were to later develop in this enclosing membrane, and without utilizing 

 the full depth of the tissue, as is the case in Scorpaena (Allis, 1), it 

 would seem as if the bone might either enclose the artery, or develop 

 either external or internal to it, without altering the fact that the 

 artery continued to lie in the cranial wall and not in the cranial cavity 

 proper. When the artery reached the opticus foramen, it would then 

 there naturally still lie outside the cranial cavity, and would there 

 give ofif its retinalis branch. The orbito- nasal branch of the artery 

 would naturally not be included in this operation, excepting in that it 

 would traverse, or be slightly pulled into, the orbital opening of the 

 myodome; otherwise retaining its original, extracranial position. This 

 supposition brings the apparently radically difl'ering courses of the 

 carotid in the two groups of fishes into perfect accord, and if it be 

 the explanation of the manner in which the conditions in the Acantho- 

 pterygii have arisen, the one or more branches that go to the eye 

 ball might have been at times enclosed in the skull wall with the 

 cerebral trunk of the artery and at other times left outside the skull 

 as branches of the orbito-nasal ; this depending upon a slight shifting 

 of the points of origin of the eye ball arteries from the carotid. These 

 arteries, excepting only the retinalis artery, apparently vary greatly 

 in this respect in Telöosts. What would seem to be their homologues 

 in the Loricati are even said by Allen to cease to be branches of 



