The Latero-Sensory Canals and Related Bones in Fishes. 485 



prei^sions extends through the related dermal bone down to the under- 

 lying cartilage, and the nerve that supplies the group here reaches it. 



One of the four groups on the dorsal surface of the head forms 

 a considerable line that lies dorso-mesial to the eye, along the suturai 

 line that separates the so-called frontal and nasal bones from the so- 

 called prefrontal and postfrontal. Many of the surface pores of the 

 supraorbital canal lie in the midst of the organs that form this line. 

 A second and shorter line lies antero-mesially to the first line, along 

 the suturai line that separates the so-called nasal from the next antero- 

 mesial but unnamed scute. A third group or patch, rather than line, 

 lies on the suturai line between the squamosal and parietal. It strongly 

 recalls, in its position, the middle head line of pit organs of Amia 

 and teleosts, and I was inclined to consider it as the homologue of 

 that line before the short parietal canal had been found, first in 

 Scaphirhynchus, and then, by filing down the bones, in Acipenser. A 

 fourth patch lies on the lateral extrascapular, in the position shown in 

 the figures. It somewhat recalls, in position, the posterior head line 

 of pit organs of Amia. 



Scaphirhynchus platyrhyyichusi. In this fish the canals agree so 

 closely with those in Acipenser, and they are so fully shown in figures 

 40 — 45, that no detailed description of them is necessary. The number 

 and position of the sense organs in the canals could not be determined 

 in the one specimen I had. As seen in the figures there is no eth- 

 moidal commissure in this fish, as there was in Acipenser, the anterior 

 end of the main infraorbital canal being represented in what apparently 

 is the homologue of the short branch canal given off in the antorbital 

 bone of Acipenser. A group of small dermal plates, on the dorsal 

 surface of the anterior end of the snout, each plate bearing a curved 

 tooth directed backward, separated the anterior ends of the canals of 

 opposite sides. 



The preopercular canal, or vertical cheek line, whichever it be, 

 traversed, in my one specimen, seven small dermal plates; and immedia- 

 tely anterior to the middle plate of this series there was another 

 plate traversed by another short canal which was horizontal instead 

 of vertical in position. This latter short canal recalls strongly the 



