136 EDWARD PHELPS ALLIS, JUN. 



dorso-postei-ior head arises from tlie side wall of the skull 

 immediately dorsal to, and contiguous with, the dorsal edge 

 of the trigeraino-facial foramen. It there lies ventral to the 

 surface of origin of the rectus inferior, and anterior to that 

 of the rectus superior. The muscle did not have a straight 

 course in my sections, but this may have been due to the 

 action of reagents. When near the eyeball it turned for- 

 ward, then directly outward to the eyeball, and then back- 

 ward around it. 



The obi i qui superior and inferior arise close together, 

 at tlie anterior end of the orbit. The superior muscle arises 

 from the orbital wall immediately dorsal to the orbital 

 opening of a canal by which the ophthalmicus profundus 

 traverses the antorbital process. The inferior muscle arises 

 immediately posterior to that opening. The latter muscle 

 here lies immediately internal to the anterior end of the 

 muscle Add|3 of Vetter's nomenclature, and immediately 

 dorsal or dorso-anterior to the orbital opening of a large 

 canal that extends from the front end of the orbit into the 

 hind end of the nasal capsule. 



This latter canal transmits a large branch of what Gregen- 

 baur (23, p. 77) considered, in adult selachians, as a peri- 

 orbital lymph sinus. Parkei" (47) has, however, since 

 described, in the adult Mustelus antarcticus, a large 

 venous orbital sinus that would seem to be the same vessel that 

 Gegenbaur described as a lymph sinus. In my embryos this 

 sinus is certainly a part of the venous system, and if both 

 lymph and venous sinuses are here found in the adult, they 

 must both have been cut oif from this blood sinus of embryos. 

 That this is what probably takes place seems shown by the 

 fact that, in Amia, the peri-orbital sinus is certainly a lymph 

 one, and that it has the same position and relations to the 

 other orbital structures that the blood-sinus of my embryo of 

 Mustelus has. From the anterior end of this sinus, in 

 Mustelus, the large branch above referred to arises, and, 

 traversing the canal in question, enters the hind end of the 

 nasal capsule. Froni there it sends a branch downward 



