480 Edward Phelps AUis jr., 



the mesial edge of the bone. As the canal traverses the bone it sends 

 a short stout branch backward and laterally along the dorsal corner 

 of the lateral edge of the rostrum, this branch having the same caliber 

 as the main canal, and traversing one or two little ossicles after 

 it has left the three-armed bone. No sense organs could be recognised 

 in this short branch, nor could any nerves be found that seemed to 

 go to it. But I should nervertheless hesitate to considered it as a 

 simple primary tube, partly because of its general appearance but 

 mainly because it occupies so exactly the position not only of the 

 anterior end of the infraorbital canal in Scaphirhynchus, but also of 

 that of the corresponding end of the same canal in the figure that I 

 gave, in an earlier work [8, p. 118], purporting to represent an assumed 

 projection of the lateral canals of Mustelus on to the head of Amia. 

 If it be the homologue of the canals in these two latter fishes, it must 

 represent the anterior end of what, in my nomenclature, is the sub- 

 orbital part of the main infraorbital canal; and the short commissural 

 canal must be the homologue of the ethmoidal commissure of the bony 

 ganoids. This commissure would, however, here arise from the sub- 

 orbital canal distal to the most anterior sense organ of the canal, 

 instead of between the first and second organs of the canal, as in Amia 

 and Lepidosteus. The three-armed bone that here encloses the canal 

 would then be an antorbital bone. But, however this may be, we 

 certainly have, in Acipenser, an ethmoidal cross-commissural canal. 



Having left this three-armed and probably antorbital bone, the 

 main infraorbital canal of Acipenser runs downward across the lateral 

 edge of the rostrum and then turns backward along its ventral surface, 

 there lying close to the lateral edge of a broad flat ridge of dermal 

 scutes that cover the intertrabecular ridge of Parker's descriptions [59]. 

 Having reached a point slightly anterior to the mesial tentacle of its 

 side, the canal turns directly laterally, passes beyond the lateral tentacle, 

 and there turns backward and laterally, in a curved line, until it 

 reaches a point near the lateral edge of the ventral surface of the 

 head and directly lateral to the mouth. The canal here has just 

 entered the triangular, lower postorbital bone of Parker's descriptions. 

 Anterior to this bone the canal had traversed, on the right hand side 



