226 



evident, for there was not, in any description of the fish that I could 

 find, any indication of a suitable foramen for its passage. Now that I 

 have found the vein with its own special foramen, I consider the con- 

 clusion as fully established ; for if this foramen, lying ventral to 

 Traquair's foramen 3, were to be greatly enlarged, it would become 

 the orbital opening of a space corresponding strictly to the myodome 

 of teleosts. But the conditions are obscured by the following several 

 facts: that nearly the whole post-opticus portion of the sphenoid bone 

 of Polypterus corresponds to the hind wall of the orbit of a typical 

 acanthopterygian fish, and not to a part of its mesial wall ; that there 

 is apparently no basisphenoid bone, either as an independent ossifi- 

 cation or as a part of the sphenoid bone, and apparently also no 

 prootic ossification ; that there is no outer leg, or pedicle (Allis 1897), 

 to the alisphenoid portion of the sphenoid bone, — assuming that the 

 bone contains an orbito-sphenoid element, which seems questionable ; 

 and that that part of the orbital wall that lies between what I take to 

 be Traquair's foramina 5" and 5'" is a part of the inner wall of the 

 trigemino-facialis chamber, the outer wall of this part of that chamber 

 being wholly wanting, or obscured because of the origin there of the 

 muscle called by Pollard the pterygoid division of the adductor 

 mandibulae. Further study of this region will doubtless make it 

 possible to determine positively just what the homologies of the sphenoid 

 bone are, but whatever they may be will not alter the fact that 

 Polypterus presents conditions from which the myodome could readily 

 be developed. A simple enlargement of the eye, by bringing the point 

 of insertion of the recti muscles back toward the hind end of the bony 

 orbit, would probably lead promptly to the development of a myodome. 



Polypterus, it may be stated, gives no indication of ever having 

 possessed a myodome and then lost it, the conditions in this fish 

 having been quite unquestionably derived directly from those found in 

 elasmobranchs and the cartilaginous ganoids, where a myodome has 

 evidently never been developed. And from the conditions in Polypterus, 

 not only could the myodome be readily derived, but also the condi- 

 tions found in Lepidosteus and the Siluridae, where is no functional 

 myodome; these latter fishes probably never having had a myodome, 

 as fully set forth in my work now in press. 



Palais de Carnoles, Menton, July 16, 1808. 



Literature List. 



Allen, W. F., 1905, The Blood-Vascular System of "the Loricati, the 

 Mailcheeked Fishes. Proc. Wash. Acad. Sc, Vol. 7. 



