The Latero-Sensory Canals and Related Bones in Fishes. 419 



ment is found, the supraorbital canal here running forward and 

 laterali}', without bend, between the nasal apertures. In teleosts and 

 elasmobranchs, on the contrary, the canal does not pass between the 

 apertures at all, turning, in elasmobranchs, backward and laterally 

 morphologically posterior to the apertures [8, p. 118], and, in teleosts, 

 either ending mesial to the apertures or turning laterally and back- 

 ward anterior to them. Batrachus tau [21] presents an apparent ex- 

 ception to this rule, but this exception is only apparent, for while the 

 anterior end of the supraorbital canal there certainly turns laterally 

 between the two nasal apertures, the anterior end of the supraorbital 

 sensory line, which is here represented by a single surface pit organ, 

 does not turn laterally with the canal, but continues forward mesial 

 to the apertures. 



The OS terminale, accessory nasal and nasal bones, together, of 

 Traquair's descriptions of Polypterus, all three of which bones are 

 developed in relation to the supraorbital canal, certainly represent the 

 single nasal bone of Amia, as I have already shown [6, p. 442], and 

 hence also that of Lepidosteus; and this nasal bone, thus constituted, 

 bridges, in each of these three flshes, the space between the two nasal 

 apertures. In teleosts the nasal bone does not usually, if ever, bridge 

 this space, lying mesial to both apertures and frequently being re- 

 placed, between the apertures, by a "Nasenflügelknorpel" [63, p. 221]. 

 Whether there are exceptions to this rule, or not, I can not determine 

 from the literature at my disposal. In Erythrinus and Sarcodaces, 

 where the nasal bone is exceptionally large [64, p. 32], there is a 

 process of the bone that projects laterally in the roof of the nasal sac, but 

 whether it lies between the nasal apertures, or mesial, or anterioi- to 

 them does not appear from Sagemehl's figures or descriptions. In any 

 event, the nasal bone of all those teleosts in which the latero-sensory 

 canals have been traced is not the exact equivalent of the nasal bones, 

 as above defined, of the bony ganoids. The three bony ganoids also 

 agree in that there is, in each, an anterior, ethmoidal commissural 

 canal connecting the infraorbital canals of opposite sides of the head, 

 this canal being found in no other bony fishes, so far as I can find 

 described, with the apparent exception only of the Muraenidae. In 



k 



27* 



