MUSTRLUS L2EVTS. 117 



the cheek of Chlamydoselachns, that is, the spiracnlar, gular, 

 jugaljand oral, represent the preoperculo-mandibularcanal and 

 the vertical cheek line, mandibular line, and gular line of pit 

 organs of Amia. The accord is much too evident not to warrant 

 the supposition. The organs of the line of Mustelus may, 

 accordingly, represent a condition of surface sense organs on 

 the border line between terminal buds and ampullary organs 

 on the one side, and pit organs and canal organs on the other. 



Four other pit organs were always found in all my larger 

 specimens, two on each side of the top of the head, slightly 

 anterior to, and on either side of, the endolymphatic pore. 

 These organs, on each side, lie in the line produced of the 

 five lateral ones of the curved supratemporal line of six 

 ampullary pores, and they may, perhaps, represent two 

 ampullae of that line that have retained their embryonic 

 place and condition. The fact that the mesial one of the 

 six ampullary pores does not lie in this same line seems, 

 however, to indicate that we have here to do with a different 

 class or group of organs, and that the two pit organs more 

 probably represent one of tlie head lines of pit organs of 

 Amia. In Chlamydosolachus, according to Clarman, the 

 supratemporal cross-commissure of the lateral canals has 

 the position relative to the endolymphatic pores of these 

 four surface organs, and not that of the cross-commissure 

 of Mustelus and other selachians. This would be easily 

 accounted for on the assumption that the two organs in 

 Mustelus are lateral sensory ones, and became, in Chlamy- 

 doselachus, enclosed in a canal. 



Other pit organs are found in Mustelus, irregularly ar- 

 ranged on the body of the fish posterior to the supra- 

 temporal commissure and dorsal to the main lateral line. 

 One line of these organs lies directly superficial to the lateral 

 canal, a position that seems to preclude its being in any way 

 directly related to the organs of that canal. Certain of the 

 organs undoubtedly form the line of ''pit organs" that 

 Ewart (18, p. 81) describes in Mustelus, and which he says 

 are innervated by a branch of the nervus lineoe lateralis vagi. 



