434 Edward Phelps AUis jr., 



parietal, but not, as Sagemehl's descriptions would lead one to sup- 

 pose, entirely in that bone. It begins, in the mid-dorsal line, at a 

 single pore that lies immediately superficial, or slightly posterior to 

 the point where the hind edges of the parietals of opposite sides 

 unite. The canal from there enters the so-called parietal, on either 

 side, and curves laterally and latero-posteriorly through the posterior 

 part of that bone, leaving it near the lateral end of its hind edge. 

 It then enters the extrascapular, and continuing its course in that 

 bone joins and anastomoses with the main infraorbital nearly opposite 

 the extrascapular infraorbital organ. But one sense organ could be 

 found in the canal on either side, and it lies in the so-called parietal. 

 In both Hydrocyon and Alestes two organs were found in the canal on 

 each side, one lying in the parietal and the other in the extrascapular, 

 and it may be that a second organ was here overlooked in Macrodon. 

 The one organ that was found in the latter fish is innervated by a 

 branch of the nerve that innervates also the one extrascapular infra- 

 orbital organ and the posterior squamosal organ. This innervation, 

 so exactly similar to that in Scomber [11], seems of itself sufficient to 

 establish that the commissure is the homologue of the supratemporal 

 canal, and not of the middle head line of pit organs, but it would 

 have been much more satisfactory had I been able to also find the 

 latter line of organs. But no pit organs whatever could be found. 

 I, however, consider the homology unquestionable. That homology 

 being accepted, a comparison with Polypterus shows, in a manner 

 that seems to me most strikingly conclusive, that the homologue 

 of one or both of the two mesial extrascapular bones on each 

 side of the head of that fish, has fused, in Macrodon, with the hind 

 edge of the parietal, thus producing a parieto - extrascapular. In 

 Alestes the anterior edges of the fused extrascapular bones are 

 distinctly evident. In Macrodon they are indicated by a faint line 

 only. The extrascapular in each of these fishes, and hence pro- 

 bably in all the other Characinidae also , in thus not pi-operly des- 

 cribed as ''ein kleiner, schupp enförmiger Deckknochen, welcher über 

 dem Eingange zur Temporalhöhle gelegen ist, und über den nichts 

 Besonderes zu bemerken ist" [64, p. 48]. 



